Bret, the closest 289 HiPo car I can recall owning was a Dearborn built convertible with a schedule date of 15F (June 16, 1964). However the engine, which was original to the car with matching VIN stamp, had an assembly date stamping of 4G14N (July 14, 1964), almost a month later. That car's block VIN stamp was in the usual place near the front of the engine. So this car was likely built a month beyond the schedule date, possibly due to engine availability, and is about two weeks prior to the schedule date of your fastback.
It would be interesting to know which assembly plant built your fastback and if the engine assembly date is close to the scheduled build date of the car. It could be a matter of the assembly plant being initially uncertain about where to stamp the block VIN, as the HiPos were the only engines to be so identified in the 1965 model year.