As T_T aid - its a rifle built at Fazakerley in March 1943, with a bolt that seems to have originally been on a Maltby. The "67 Mk2" indicates that the cocking piece was made by Singer Manufacturing Co, Ltd Clydebank. There were over a 100 subcontractors making parts for No4s in UK and North America, and most of them added their identity code to the part - eg "N67".
If you post some pictures, there is often a lot more history evident in other markings or in the way the rifle looks.
There were three British factories making No4s - the Royal Ordnance Factories (hence "ROF") at Fazakerley and Maltby, and the private BSA factory at Shirley. They developed a coded numbering system - numbers starting with a "1" were assigned to Maltby, "2" was Fazakerley, and "3" was BSA. In front of the number they added a prefix - starting at "A", going through the alphabet, and then starting again with "AA", and so forth. 4-digit numbers are very early BSA rifles.
The suffix "A" on a rifle number was added to indicate that one or more of the parts had to be hand-fitted, instead of just being bolt- together. Usually something very minor, and just an indicator for a future armourer.
The majority of rifles held by UK at the end of WW2 went through a big refurbishment programme in 1949. If the rifle has anything like "FTR" on the side, or if the woodwork is beech, then its most likely been refurbished in service - this is often where the "A" suffix was added to the number.
Because your rifle has a "mismatched" bolt, this is something that happened to it after it left UK service. No4s served with about 100 countries, and then thousands of civvie owners. Very common for bolts to get swapped around. In theory the bolt should be fitted to the individual rifle (ie in the lug bearing surfaces) but in practice No4s are very tolerant of bolt fitting and headspace.