best size rear tires for my centerlines?

[NorCam]

Pro Tracs were around in the mid 70's but I'm not sure exactly when they came out? I used to see a lot of them in the late 70's and the other premium street tires of that era were the original M&H Racemaster and McCreary's. They were the shiznit back then and all of them had N50/15 and/or L60/15 bias ply street tires.

Maybe someone can chime in that knows exactly when they started making them? Here's a shot of an original pair of Pro Tracs



Earliest ad I could find looks like early 70's perhaps