Agree with all of your thoughts Les.
Not a nuclear sub. the tankard has the feel & patina of 1940/50s, sorry should have given that last fact in the original.
I suspect as you do that this is a "wind up" - but I would like to get to the bottom of it, if I can then the tankard and the story can be returned to the ship in question or to a suitable museum (after I have a had a few pints out of it at a few suitable festivals.
Rgards Skippy