Did all 69 302's have a vin stamped at the rear of the block ?
As always there were likely mistakes that might have gotten past the inspectors or as you stated it could be a replacement/service especially considering the high failure rate of these engines though I'm pretty sure the piston problem would not have been know in April, since the assembly of the cars started in the last weeks of April) there still would have been so engines needed in case there was a need similar to that in other engines
The plants had been stamping all engines for over a year so its not likely that the number of missed (if that is the case) was very high but the determination of it were a replacement/service item would come down to comparing casting, machining, and assembly dates on all the parts compared to the cars real build date. If any of these dates are after the car was built then you have the most likely answer
An interesting but likely impossible task would be to collect all know service block dates since these engine may have been purpose built at specific time period and put into the supply system. Of course if the engine plant just added to the normal build process a few engines every day or week for the supply system this effort would not be useful. Just a thought.
I do have a similar non-VIN block cast 9C for a 9D car that I believe is original to the car
