Another reason for the dorsal filet was a disturbing tendency for the plane to enter a high speed stall at higher speeds, such as diving and recovery from dives.
The Jug pilot I talked to said they lost a guy that way, no one could figure it out, so they went and "recreated" the man's last known actions......
When "my" Jug driver started to pull out from a dive, the Jug immediately flipped over onto it's back with no warning. He said he tried it again, and the plane always flipped to the left, absolutely no warning at all. His CO went up and tried it, same result. Mind you, they were in Jugs in clean trim, not loaded to the gills with ordnance. That's probably the only reason they survived and the other guy didn't.
To cut to the chase, their findings went up the chain of command, a Republic field rep was notified, and the dorsal filet "fix" was added.
FWIW, he said his D-20 was retrofitted with the filet after they found out what was wrong, as were all other bubble-top Jugs prior to the D-30's factory build with the filet.
I kinda doubted his story when he told it to me, then he showed me several pics of his plane. You could see the D-20 minus the mod, and the modified version of the same plane. He said after the filet was added, the plane was rock solid in dive recoveries.
Trivial info, I know, but this discussion got me to looking at my notes on him. Sorry to butt in..........