You posted a pic of the RH dropout, if there was a serial no. on the dropout it would be on LH side. The pic you posted of the grass has some blurry tubular thing in front of it.
Check all around the fork steerer, the stamp may be light.
I'm not sure about any track tandems as those were not in the catalogs but Schwinn changed from a single-sided drive TA crankset to a crossover drive Campagnolo crankset in 1977, which also allowed them to go from a double (10-speed) to a triple (15-speed) chainring setup:
That is very cool! It makes sense that the forces would be more evenly distributed with a crossover drive.