Well, Woody...
I haven't begun looking through my own stuff in earnest yet, but I did my usual snooping on the net. I think I exhausted just about all my regular avenues. I'm gonna have to break into that list of search engines I have.
Anyhoo, I talked to a pal at our Saturday Night Mafia's Groovy Get-together at Guido's and he is only aware of one book on the P-66. Also, he has had little luck in web searches. But I ain't givin' up yet!
As far as the possible similarity of the P-66 in this regard to other Vultee aircraft, it hasn't escaped me. My serious research efforts usually are broad and wide!!
Fade to Black...