No.1234 was forever No.1234
Didier, the photos in your post #26 show that the number was re-stamped, with the same number as before, over the original marking, which was obviously judged to be inadequately legible.
As the No. 8s were apparently made using surplus No.4 Mk1 and No.5 receivers, i.e. parts picked out of a bin of manufacturing overrun components, there is not going to be any simple correlation between the original date of manufacture and numbering of the receiver and its later use in constructing a No.8. There was apparently no deliberate FIFO selection!
Furthermore, Peter Laidler's contributions have said (more than once, in response to repeated queries over the years) that "the receiver was the rifle". I.e. it never changed its identity, and was thus never available as a spare part. If it was made as a No.4 receiver number 1234 in 1949, then it remained number 1234 even if it was dug out of the overrun surplus bin and used to make a No.8 in 1954. If the receiver 1234 was scrapped, then rifle number 1234 was gone forever - there would not be a replacement number 1234.
In other words, the receiver number on your rifle will not have been altered, regardless of what other stamps you may find on the rifle.
Please refer to Peter's exhaustive contributions on this and other themes to confirm the above. I may be wrong on this topic, but I doubt that he is!
Patrick