I've seen many different types of stamps on pistols done by unit/base armories. Often called ''rack'' numbers , these were stamped to coincide with the racks in the armory for quick-look issue and inventory purposes.
Ah, one of the famous (or infamous) 'Balimoy' slides. They were late 70s to early 80s contract slides made by DSE Inc, of Venice, FL. They 'did business as' (DBA) Balimoy. They were the first replacement slides that I saw that were not made by one of the common manufacturers (Colt, Ithaca, Remington Rand, etc.). Decent slides, for all that. I have never seen a strictly U.S. military 1911 stamped like that (J321), but that doesn't mean it wasn't done somewhere, somewhen. I would guess that your pistol was seconded to either another country or perhaps a Law Enforcement Agency that applied those stamps.
For me - that is pretty cool (I like Stamps LoL!). Here is my SWAG: some US LE agencies obtained 'free' 1911s from DRMO and maybe they marked them? 2cd SWAG: it is a rack number from a US Navy unit? Try a FOIA on it: some of the guys getting CMP 1911s have had modest success obtaining at least a partial look at where their pistol has been over the years.