Does it match the size of the yashicamat stuff you have for sale? That is the smallest size of bayonet, the Bay 1, there are also bay 2 and 3. The oldest 3.5s seem to take the Bay 1, the other 3.5s seem to take the bay 2. Bay 3s are used by the 2.8s. I am guessing from the size that it just might be a bay 3, but I can't tell from here. I got one of those in a box of "junk" that I picked up on eBay once and it sold for over $100. It was in absolutely mint condition with the leather case, however, so one that looks more used and does not have the case will probably be worth somewhat less. (But still way more than it should be ) Sometimes the size is listed on the mount, is there any lettering there?
I would take a look at the Global Rollei Club at http://rolleiclub.com/. They have a listing of all of the TLRs and which accessories they took, along with lots of other good stuff.