If you look carefully at the original picture posted, I don't think that it necessarily is from a PBY Catalina. I don't think the rivet pattern around the supposed blister location actually matches the way the blisters were riveted.
There is a nice series of 176 photos here of a PBY walk around. Pictures of the blister appear on pages 9 and 10.
http://www.net-maquettes.com/picture...-5-a-catalina/
This period photo appears many places on the net.
https://upload.wikimedia.org/wikiped...ne1a34894v.jpg
I think a close study shows a different rivet pattern than on the recovered piece.
Do you have a link to an actual PBY photo that shows a good match?