Wow, an eight year old thread and no one suggested the most plausible answer. Schwinn stamped Men's Frame Heads, and Ladies Frame Heads. The Ladies heads were only in one height. The Men's were stamped in several different heights (17&20", 22", 24", 26" frame sizes). The machining, drilling, and stamping were done on the loose parts "before the head was welded onto the rest of the frame. My thought is, this was "just easier", likely faster. If they broke off a drill bit while drilling the name plate holes, just toss the part, save the production time, they had another 1000 frames to build "that day".
Out of all the examples you have presented of "upside down stamped serial numbered frames", NOT ONE is on a ladies frame? The men's frame heads are symmetrical, and it would be very easy to stick the part into the frame welding fixture "upside down". The stamping is not upside down, it's the frame head welded into the frame "upside down". It would be impossible to weld a ladies head upside down, and that's why you have no examples.
The stamped frame serial number was not a 17 digit DOT Vehicle Identification Number. It was just a number that helped a dealer, and a customer have a way to help identify the bicycle from another one like it. The dealers and sometimes Police Departments used the numbers for registration purposes. Schwinn did not hold the stamped numbers in any great importance. When the potential for CPSC product recalls came up in the 1970's, Schwinn adopted the date of manufacturer system by stamping the date on the name plates. If Schwinn placed the importance of having the exact correct dated stamped serial number on the frame there would have been no reason for the financial expenditure of adopting a stamped name plate program. Any Schwinn Dealer of the 1970's will confirm that when they placed their orders, they received a confirmed shipping date. They routinely ran at three to four weeks out. That means they knew, and planned for how many bikes were going to be built on any particular day "ONE MONTH" from the date of the dealer order. The guy that stamped the serial number knew how many he had to make, call it a quota for the day. The next guy that placed the finished heads into the frame fixture had the same quota number for that day. As a Schwinn Dealer on the other end of that pipeline, I had already sold and taken deposits for the bicycles that had not yet even been built. There was pressure to hit the quota number in order to make everyone happy.
John
