hi there,

as you said, there are probaby thousands of cultivars and it's hard to ID begonias sometimes, based on that fact alone!

there are a few good online resources. if you browse old posts, there are links to some of them. one of the best places to learn about begonias so you can start describing them more accuratedly is the american begonia society homepage and their virtual greenhouse.
the houston branch, linked below, has a good pictoral database for both species and cultivars.

as far as "vintage" plants, if you can get a hold of Mildred Thompson's begonia bible, simply entitled "Begonias," there are lots and lots of pictures in that book, which is no longer in print. if you're lucky, maybe your local library has it.

lastly, try posting a picture of it in the gallery for ID! HTH,

-ming

Here is a link that might be useful: Houston Begonia Branch