I just read an article in a 1970 Schwinn Reporter (I don't remember what specific issue) that described the frame serial number move from the dropout to the head tube. The article indicated that the move was done to make it easier to read the serial number, and that the change was made primarily for law enforcement because at the time bike theft was becoming a major problem.
Anyway, the article stated that during the transition there would be bikes with numbers in both places, exactly as seen on the tandem in question here. It stated that this was because when the change was made, all frames in existing inventory were simply re-stamped with new numbers on the head tube. The way it was worded seemed to imply that the complete frames with dropout stamps already existed, and they were then simply re-stamped on the head tube. It also stated that when two stamps would exist on the same bike they would be different, and that in that case the head tube number would take precedence and be the true serial number for the frame.
Taken at face value this would suggest that the head tubes were stamped after the frame was complete. In any case I thought this was very interesting. I wish I still had the article and could make a copy of it, but I only had a chance to read it one time.
Another interesting article called 1970 "the year of the derailleur", and reported the exact number of 10-speed bikes vs. Schwinn's total production from 1960 through 1970. From memory it showed that in 1960 10-speed models were only about 5% of production. This number gradually increased over the years to about 10% in 1969, and then instantly doubled to 20% in 1970. At the time they projected 44% of the lineup would be 10-speed for 1971. All in all very interesting reading. I wish we could get these Schwinn Reporters scanned and online!
