'PA' is a Scorpion serial number, so it looks like it has a Mk.1 scorpion cylinder, and as you say, would appear to be a 'Bitsa' made up by somebody, and the rear sight could have been fitted at any time.
Can't explain the lack of stampings on the barrel, which does seem strange as the 'PA' will have been stamped on the linkage part of the breech.
Does it have the standard Buccaneer stock, only in black ?