Panzerwulff, I also found a reference to the K gun here: http://www.warbirdsresourcegroup.org/NARG/catalina.html
It looks like the RAF and probably the RCAF had them with twin guns in the blisters. Do you have the Squadron/Signal book on the PBY Catalina? Do you know if there are any good pics in there?
This is the pic I found the other day. You can see the drums, but beacuse it is on two levels, I know it is not a PBY, maybe a Sunderland?
<<EDIT>> yes the pic is from a Sunderland, I found an aircraft website with a very similar pic showing the interior and the caption said it was a view from the tail looking forward. Sorry to get your hopes up. I wonder if Dave has seen pics like this before he closed up his sunderland...