Tom - I thought your bike has the drum rear brake? If so, it's either been changed from the disc the S, RS and RT came with stock, or was a T to begin with. The VIN should be stamped in the right front steering head gusset and it'll tell you what the bike was originally.
Whoever you spoke with at Corbin didn't know the BMW airheads. Your seat was used on the R90S (74-76), 100S, 100RS, RT, and finally all of the models after 1981-84. It's usually known as the S seat, though it's the same one used on the twin shock RSs and RTs.
I like the look of your bike too. It's been well loved!