The USS Irex SS-482: Philatelic cover

This is a philatelic cover, but it is not a “first day cover“ because this Armed Forces Reserve stamp was issued on 21 May of the previous year.

Rather, it is a philatelic cover intended to commemorate some event that took place on 26 December 1956 and presumably had something to do with submarines. However, my brief search turned up no such event.

Someone troubled to mail this commemorativee cover with the Irex return address. I've no idea whether all boats were expected to commemorate the event in this way or if the commemorated event had something to do specifically with the Irex. If anyone has an answer to this puzzle, I'd much appreciate knowing of it.