You can eliminate 1960's, for no bike that I have stripped for parts has dated back from then. The 60's examples also used the red-dot Q/R setup, rather then the tab at the top. From your inputs, I am inclined to believe the bolder stamping came later on in the run.
I know at least one of them was the original off the '71 Schwinn Continental - I am putting a guess to it that it is the one on the left side, but I do not recall exactly.
I've seen the drilled variants, I've seen variants with a sharper angle to the side of the lever bodies, vertical drillings as used on the Raleigh International, etc. They must have made at least 20 distinct variations in the basic design, let alone this small revision to their original design.
-Kurt
__________________
