True -- ca. 1960 Capo frames had 5-digit serial numbers starting with a 4 (see my signature). Production run rate, according to Harald Cap, Otto's son and one of my contemporaries, was about 5K units/year.

This leads to another sheer speculation on my part -- I wonder if Otto Cap ever built frames under relabel contract to other companies. All of us have seen numerous examples of similar cross-labeling of marques among other framebuilders.

Given the top tube loops and the change in seat tube clamp, I also wonder whether this is simply a late 1960s, even 1970 (hence, the leading 0 in the S/N) model. (Otto Cap would have been running out of 5-digit serial numbers by then, and the new decade would have been a logical time to switch to a new numbering system.) The head lugs and the trim pieces on the seat stay caps, the fork crown, the rear dropouts, and the placement of the serial number shout "Capo."