Yup, Stephen K nailed it. Dale has also shown that many museum tsuba are casts but dating them, and guaranteeing they are actually old, remains the problem. Some museum pieces probably contain provenance information but I'm not aware of any invasive or non-invasive means of dating any tosogu or nihonto.
These are salmon returning to freshwater when the tips on the jaws grow inwards to form hooked jaws. This would be the most commonly encountered form that anyone but fishermen would be familiar with. Not usually the best eating but still pretty good with sufficient soy and sake. At least 1 of the dragon fish shown above have jaws that flare outwards.
If you live in the U.S. there are several Japanese sword shows per year. Nothing is better than holding and examining tosogu in person and in the company of other collectors and practitioners. Bring an optivisor or equivalent.
Roman is still doing restoration work and apparently some complete tosogu. Follow the link above and you'll see what he has done with iron tsuba restoration.
There's a company in NZ that sells pieces or sheets of the abalone nacreous layer. This material was (is?) also used for tosogu and I recently bought a fuchi/kashira set from an NMB member. The Met in NYC also has a tsuba, possibly made by the Somada School. The method is known as raden maki-e.
We mailed stuff back to the U.S. from a post office in Kyoto. The postal worker actually trotted to the back of the store and brought us a box. In the U.S. you're lucky if the postal worker doesn't go on break. I've received swords from Japan in less time than I have from Cincinnati.
You should all know that Markus has a bound version of his 10 years of articles on tosogu and armor (267 pages) for sale now. Order directly from Markus.
Good question! I've been getting dispatches from the field for several years but I don't recall how. I tried calling the Met and got a garbage recording, so I sent an email asking how. No response yet. The full title is Arms and Armor, Dispatches from the Field, Curatorial. The latest is- Kao: the monogram of Japanese sword fittings makers. You can also search all previous curatorials. When/if the Met gets back to me I will post it.
If you don't already subscribe (free) to the Met's Arms and Armor Dispatches from the Field you're missing some very good articles on Japanese arms. The latest (#23) from Markus is- Kao: the monogram of Japanese sword fittings makers.
armsandarmor@metmuseum.org