OK, I read this thread pretty fast, so maybe I have it wrong here.
It can't be a situation of using up old 4506 no dash frames since the no dash frames are all stepped. His frame is not stepped, so it has to be the newer 4506-1 ( 1006 style frame) ,
So, it looks like a 4506-1 gun with a squared trigger guard. Could it be an early production 4506-1 that had the squared 4506 trigger guard or could it be a custom trigger guard on a 4506-1 that someone had done?
Oh, and all black triggers and hammers are MIM. The older 4506 and older 4506-1 have the flash chromed triggers and hammers, the newer 4506-1 have the black MIM triggers and hammers