I am not sure how you dated this, but this was made by AMF Roadmaster, the same AMF that also made bowling equipment. They were decent bikes but awfully cheap. Compared to a Schwinn, Huffy, Columbia, or an English road bike of the time, they wore out pretty fast. Good thing is that many critical parts were shared among other manufacturers, like that shifter cable and hub. However, compared to the garbage today at big box stores, they have held pretty well. All these old bikes have.
Anyway, AMF serial numbers are VERY hard to decode, nobody has cracked them yet. I can tell you that bike is actually made between 1971-78ish. The give away is that government-mandated safety tag that reads “BMA/6”.