Regarding the bolt number. Have a close look at the rear face of the bolt handle. Normally the serial number would be stamped there; not on top as per your photo. (At least both my Maltby made No4's are at the rear).
So maybe that number is something else.
Also, I'm not sure if the 300/600 flip sight would be original for a 1942 rifle. Hopefully one of our more knowedgeable members can confirm!