Sorry if this is supposed to be common knowledge, but when did Estes stop making the "s" (short) motors and start the "T" motors?

I see that there is a mini brute sprint (up for auction on e-bay claiming to be 37 years old. It is the "t" motor version (1215 K-15). I flew the "s" motor version (K-15) 38-40 years ago.