Unfortunately Rick, I can't help you with a pic of the 20mm on
Monssen's stern, as as you saw, she is
way too deep for scuba divers. Deepest wreck I dived / has been dived there in IBS, is USS
Atlanta in 130m/430ft (we were first to scuba dive her back in 1995, and she still gets the occasional visit today). But we did sonar locate what we believe to be one of the APD's (either USS's
Little, Gregory or
Calhoun) in about 180m / 600ft of water, but did not have the technology (or the cojones) at the time to attempt to dive it, as we were still using open circuit scuba then. Once closed circuit rebreathers became common for deeper diving - and I used one myself till I hung up my fins recently - it is certainly a diveable wreck, but for very
very experienced divers only of course. However, first someone has to relocate it, dive an ID whatever wreck it was we located back in circa 1997. I just never got back to Guadalcanal after I moved my focus to SE Asian wrecks, so whatever it is it still sits there undived.
Anyway, back to rarely seen 20mm's. While not on a DD, below is a pic of an oft debated one - as no 'above water' pics exist / or have appeared of same - on the very bow tip of the wreck of HMNZS
Moa (buried into the mud in Tulagi Harbour). Certainly not a standard regulation issued / installed item, but by all historical accounts, shortly before her sinking, was 'acquired' from the 'dry' USN when, greased by two bottles of gin, it mysteriously slipped off the back of a USN truck