Schwinn bikes had the serial numbers stamped on the component, BB shell, drop out or head tube prior to those parts being used to build a frame. Schwinn was doing this way back in the 30's! The date associated to the serial number is the date the serial number was stamped on the part and the date has absolutely nothing to do with any kind of build including the frame. This fact has been proven over and over again and it has even been mentioned in the Schwinn Reporters.
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Here's a pre-war Balloon frame with a pre-stamped BB where the number was welded over.
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Here's a 1970 Tandem that had two serial numbers, a pre stamped May drop out and a pre stamped May head tube. This was the time when Schwinn switched the serial number location.
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