I can't vouch for how things were done at Inglis or Enfield in the 40s to 60s but currently the frame is the master component and is the master serial. If a frame required replacement then I would expect the gun to be scrapped and parts salvaged rather than put a new frame on it.

I don't think Enfield would allow a pistol to leave the factory after FTR without a serial, even if it arrived without one. I believe this may be a xTxxx serial No2 frame that has had the number very carefully removed. There is a very shallow depression only visible when viewed with a rule against the frame (in the red box in the photo). The Blue box highlights where I'd expect the number to have been but it is straight. The frame has no slot in the grip for the stock.