That is more than possible, in fact it was standard procedure for Schwinn in the '60s and '70s. They would stockpile parts (including frames) needed to build various model runs in advance, then build the bikes in batches once everything was acquired. One of the biggest mistakes I see Schwinn enthusiasts/collectors make is to assume that the frame serial number accurately dates a given bike. Normally there would only be up to a month or two between frame and bike builds, but that could often be longer and for some less popular models I've documented differences of more than 6 *years*. Demonstrating this is easy for '76 and later models since that is when Schwinn began date stamping bikes on the badge with the day of production. Accurately identifying when '75 and earlier models were produced is possible but it requires more intimate knowledge of the model and original components present.