The serial numbers starting in 1937 on the one piece crank models were machine stamped and there have been stamping issues a few times over the years. Either a machine malfunction or a worker's brain fart setting up the stamping dies.
The crank casting date and the fork casting date should verify a year of production. On the inside of the left fork leg just above the axle slot there should be a two or three number stamping. 9+6 = Sept. 1966.
Here's a funny. The serial numbers were stamped on the component prior to that piece being used to build a frame and in May of 1970 the serials were moved from the drop out to the head tube. Here's a tandem that had two serial numbered components at the time the serials were relocated.
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