That AN/BN/CN ammo was not made specifically for the Bay of Pigs invaders. It was made around 1953 for general clandestine use. The ammo was issued to the invaders, though, and large quantities were released on the commercial market, primarly through Interarmco, so there would be a "source" that could not be traced to the U.S. government. In the same way, the M1 rifles provided by Letterkenny were screened by maker and serial number so that they could not be proven to have been in U.S. service after 1945.
The "cover" was pretty transparent and the Cubans knew darned well who was back of the invaders; JFK, of course, never supported the idea and, feeling the cause was lost, chose not to support the invaders. (Not the first or last time the U.S. betrayed its surrogates; Afghans who helped the coalition troops were promised visas to come to the U.S. Now the administration has decided to pull out and leave them to the tender mercies of the Taliban.)
Here are the actual factories that made that ammo. The "dates" are, of course, fake, and may have been lot numbers.
AN - Twin Cities Arsenal, 1/40 - 9/40
BN - St. Louis Ordnance Plant 1/40 - 5/40
CN - Lake City Ordnance Plant 1/40 - 9/40
Jim