This is a photo of the blister on the PBY (OA-10) Vickers Canadian built plane. The plane is now based as Duxford, but I don't think the blister installation is at all original so this probably does not help us at all.
http://data3.primeportal.net/hangar/...5_44_of_87.jpg
Here is the history of the plane and you can see at one point the original blisters were removed and the blister area was filled in.
https://www.catalina.org.uk/our-catalina-g-pbya/
The 7th and 8th photos down the right side of this link show the blister area on a PBY in the National Aviation Museum in Florida.
http://ru-aviation.livejournal.com/1736093.html
Note again only two rows of rivets. The 8th photo shows the back of the blister and you can see how the flange goes around the bottom of the blister and ends at the back. But another flange appears to go over top so we should see it on both the top and bottom of the blister.