Trying to figure out what it is...
The patent was filed August 30, 1977, and was granted October 3, 1978. If that helps any.
I'm 99% sure that the first BS keyboard was used on something in the 3270 or 5250 family.
If it was first used on the 5250 line, it'll be on the 5251, the first of the line. The 5251 came out in April, 1977, but there's a time that the inventor can wait to file the patent - I want to say that it's a year.
If it was used on the 3270, then... the 3277 or 3278 will be the first with it, because the 3275 was too early - there's at least one example from 1974. And, come to think of it, the 3277 might be too early, too - I found a patent describing something else that was filed January 1976, that mentioned a 3277.
Edit: Of course, I could be completely wrong, and it be something like the Electronic Typewriter (aka Selectronic, which, along with the Wheelwriter, uses a "Selectric Touch Keyboard" - read: buckling spring.)
But I'm not sure - release dates for this stuff are a pain to find.