That is a unique piece for sure. The receiver is proper for a 48 transitional H&H; some were undrilled, some had one hole and some had 2 holes D&T on the rear bridge.
I have seen numerous M70 barrels with 5 characters (1 letter and 4 numerals) in addition to the standard marking for special orders and return repairs/rebarrels etc. 300M 48 is how a 1948 300 H&H is marked.
The proof mark is the big mystery. Is it a PW or a P? The P is a mail order replacement barrel.
The receiver has been reblued or it appears that way due to digicam images of gunmetal.
It would help to see the under barrel marking to see if it is original or redone. Look hard at the breech end and see if there is evidence on a 2d extractor cut (possibly filled) at 6 O'clock.
Also did you try and chamber 300 H&H in it? Many times smiths just rechambered the 300 H&H to 300 Win Mag without setting the barrel back.
Let me know what you find as far as my questions.
