All Z16's came with M20's with the regular 1965 Muncie casting # of 3851325. No M21's. I presume that was due to the mandatory 3.31 axle in all Z16's. And as already noted, no Z16's were built at Fremont.
Fremont built other GM A-body cars too. Maybe it came in a GTO? Fremont Chevelle VIN ending 5Z128987 was the last Chevrolet to leave Fremont at the end of February 65. Which is higher than your VIN stamp of 5Z128230. But your trans was assembled March 10, well after the Chevelle with VIN ending 5Z128230 had already left the plant back in February. So unless the VIN stamp has a factory mistake in it, it could not have come in a Fremont Chevelle. (But VIN stamp mistakes were very common at Fremont, so...)
RPO production totals for 65 Chevelles have no M21's listed. There is not even an ECL for an M21 in a 65 Chevelle. So if it had come in a 65 Chevelle it would have to have been some sort of COPO. But as to the one known 65 COPO 396/425 Chevelle, it was built at KC like the production Z16's, so not that car.
Regarding the "848" case, per
https://www.chevydiy.com/muncie-4-sp...ication-guide/ :
"Corvette production determined how Muncie design evolved. By late 1965, the 396 Turbo Jet engine, rated at 425 hp, was introduced. A very small number of 396 engines also found their way into the 1965 Impala. I learned from speaking to many builders that the rare 3864848 case seems to be in a few Corvettes and full-size GM cars with big-block engines. The 3864848 case with the last 848 numbers milled off is in some full-size cars, such as the Impala. This usually indicates that the gears used in the milled cases might be of a different steel alloy. The case appears to be identical to a 3851325 case so perhaps it is made of a different aluminum alloy. It appears that more milled cases are in circulation compared to unmilled cases. They all seem to come from big-block cars."
I have no idea how accurate any of that is, though. The second and third sentences are certainly not entirely accurate (396/425 was more of a "mid" model-year release than "late", and there were a lot more than a "very small number" of 396's installed in 65 Impalas), so I take it all with a grain of salt ...