The final component of an item's rarity, as with all collectible things, is its provenance. This comes in two forms. For example, a random helmet isn't going to be worth that much - they're pretty common. A helmet claimed to have been found on an important and historic battlefield (with proof) is probably worth somewhat more. A helmet worn by someone famous on that battlefield is going to be worth even more. Just like art pieces owned by famous people or that were publicized in some ways tend to be worth more, military commemorative items like the ones you see on this site are similarly affected by the quality of previous ownership and the origins of an item.
Some of this is tied to the previous factor - decoration. A rarer item for a rarer event or more important person also tend to be decorated better, made with rarer material, or both. Take this cup, for example:
It's an otherwise fairly unremarkable looking cup. Pewter, obviously, but only a star in the middle. The secret is the words on the back - a personalized pewter cup costs money to make, and this person, a major general at the time, was worth it. It was for the major general who was the commanding officer of the first unit that invaded China in 1931. A pretty historically important figure, at least in military terms of Sino-Japanese conflicts during the lead up to WW2. It commemorated him getting wounded. On its own, a pewter cup with just a star is nice, but with this extra dimension the cup becomes a one of a kind collectible that isn't likely to be found anywhere else.
These one-off events in the military history of Japan tend to be the most interesting for items, at least in terms of something's provenance value. For example, this trophy cup is from the training fleet's arrival at Los Angeles in 1933, one of the last trips that IJN ships made to the US in peace time.
That year Japan withdrew from the League of Nations and tension with the US only increased from then. This trophy, commemorating the fleet's visit with a sumo-wrestling match amongst the sailors, was a relic from the overseas Japanese community in LA and a very interesting historical document from the past.
The number of people involved in the events commemorated also becomes a factor in something's rarity. Lots of soldiers fought in the 2nd Sino-Japanese War (China Incident) so they're pretty common. A lot less soldiers were Special Naval Landing Forces, which makes those items rarer. When it commemorates particular battles in the 2nd Sino-Japanese War instead of just the war generally, then the item becomes more interesting still. When you add in additional personal details - a soldier's career, a unit's progression through a bunch of locations, etc, then it's pretty remarkable. This is why cups with a person's full name always meant more to me - when it's just a last name, like Watanabe, you really have no clue who you are dealing with. When it's got a full name, a unit, then this person becomes traceable, at least in theory.
The same can be said of units. Most of the time units on these cups and other commemorative items are just described with a number - 39th Infantry, 1st Imperial Guards, 9th Transport Battalion, etc. However, the Japanese actually have a habit of calling a unit by the commander's name, it seems at least in their use during operations. So for example, this cup:
Instead of having the unit number on it, it has the unit commander's name, Hasegawa. It was stationed in Manchuria. It's not always easy to track down which unit this actually is - records of commanders can be spotty, and sometimes there are multiple candidates, but sometimes it's very clear because the commander's name is unusual or if it's combined with an event - the battle of Shanghai, for example, and all of a sudden you have a very specific and identifiable thing in your hands.
Another thing worth thinking about are boxes. To the Japanese, boxes are almost as meaningful, if not sometimes more meaningful, than the item itself. For commemorative items there are usually two types of boxes - the wooden boxes, which are themselves valuable and expensive, and the paper/cardboard boxes, which are not inherently valuable but can be if it's in good condition and especially with a nice sticker on the front that still survives, giving it a nice historical value.
For example, ceramic items in Japanese culture, such as the tea ceremony culture, are usually signed on the lid of the box with the name of the item, and also a signature or stamp from the maker. This can be pretty simple, or can be very elaborate. Sometimes the items don't come with the box, but the box is ordered by the person owning the item. Sometimes that person may ask someone else famous - a literary person, a political figure, etc, to write something on the lid, maybe on the inside, to say what this item is and what its significance is. Really special items can have two boxes - the outside box to protect the inside box which is adorned with signatures of this sort, so you don't want any damage to the signed box itself. It can get very elaborate.
One example I have right now of this kind of box is this Red Cross cup:
The cup itself is a nice lacquer Red Cross Association cup. The lid of the box, however, tells you that it's a cup awarded in Taisho 5th Year (1915). Yes, it is possible that some of these could be fake - the writing could've been written on later by someone else, but to be convincing you need an item that matches the description. Also, these boxes are usually quite well made, so there shouldn't be a lot of give - the cup should fit the box fairly well. Some boxes even come with internal fitting to make sure the cup doesn't move, although those tend to be post-war items. There are also clues as to the writing style - this box's writing is clearly done with a brush and ink. I've seen ones that look like it's written by someone with a marker - that's a lot more suspect if it claims to be an old item!
The stickers on the boxes also sometimes tell a story. This cup commemorating the Shanghai Incident, for example, has a nice sticker on the back of the lid:
It tells you that instead of just distributing the cash award for the soldiers participating, the regimental commander decided that it's better if everybody got a nice cup to take home instead - money will be spent or saved, but the cup is a physical reminder of the battle they went through and will let the soldiers remind themselves of their action long after the event was over. For a cash-strapped soldier this might not have been the most popular thing, but I can see how a commander might think that. This is all written on this sticker that is probably pasted onto the back of the lid of the box for everyone's cup. It tells us why this item exists - and it's a fine piece of history! Otherwise we could only guess why this cup was made and what it was for. Things like this also help us understand how some other cups may have been made - the really special ones are very likely made at the behest of a unit or a group of soldiers hoping to remember something they did together. For those of us collecting military memorabilia, that's a very interesting additional piece of information that helps us understand our collection.
At the end of the day, more specificity is always better. A cup with a ship on it is nice. A cup with a ship on it that has a name is nicer. A cup that has a ship on it, with a name, with the sailor's name, and with the event it's associated with is even better. Then add a nice box and explanation of why the cup was gifted - then you've got a complete package. As I said when I started this blog series on rarity - if there's ever a cup from Yamamoto to Nagumo congratulating him on his victory at Pearl Harbor, signed by Yamamoto on the box and with a set of 3 silver cups each showing, say, a different carrier with planes taking off - that will be an awesome item that any collector would love to have. It probably doesn't exist, but who knows?