I can't answer your question about seat post numbers, but I can comment about the date of at least one Super Course that has its frame nr stamped on the left rear drop out--namely mine. That bike is nr 339372. She's a 1973 BR. (bright?) Green, 23 1/2" gents 10 spd model. I still have the receipt and associated paper work. It indicates that I bought her the 25th of April, 1975 as a "left over" (which goes to show that even during the so called "bike boom" it wasn't all plain sailing for some cycle shops) for $167.00 plus tax from the Cambridge Cycle Mart at 1100 Cambridge St., Cambridge, Mass. I wonder if they're still in business? I wanted the Professional model but on 2nd Class engineman's wages, the Super Course was what I could afford.
George in Maryland