A few answers....the USDA (through a program named GRIN) stored commercially available tomatoes offered by companies through the years, but it seemed as though those who have a passion for tomato history didn't know much about them until they put the collection on line about 20 years ago. And most old tomato enthusiasts focus on family heirlooms, not the authentic old US company produced varieties.
So I just came along at the right time with the right passion and curiosity, compared the old seed catalog listings with the listing in the GRIN program....the USDA had them in frozen storage, some for over 60 years. The other thing is that they used to, at least, send samples of 25 seeds of anything requested (our tax dollars and government at work for us) - so I did my research, found 150 or so historically important tomatoes that seem to have vanished from current catalogs - requested them, got them, grew them and saved seed from them.
I sent many to seed companies for reintroduction, and offered others through the Seed Savers Exchange - hope to write a magazine article about it someday, and have a future book in mind.
the best part is that some of them are really, really good!