I have seen and handled a gas port cutaway with serial number 0000000. The receiver is SA with original finish. The parts that I remember pooped out to sometime in 1944 including the lock bar rear sight. Pete Cardone, of course, thought it came from SA itself. No documentation that I know of.
I knew this rifle before I knew Pete. It was owned by a dealer in VA who used it for display at gun shows and it was not for sale until Pete talked him out of it. Pete gave $2500 and offered it to me for $3500 that same day. Later he advertised it for $7500 in his catalog. He sold it, but I don't know for how much.
I have a 900K cutaway with mixed parts including postWWII rear sight. I've had it more than 20 years and was told by the seller that it was depot made. This rifle has been re-parked.
There is or was an outfit here in NC that used to make a cutaway if you provided a receiver. I thought this current auction rifle may have been built by a private enterprise due to the type of cuts and the lack of paint on the cut surfaces of the wood.