Origins of No.4 Serial number?

It's M1501
Any thoughts on who made it? (1942) Would this be BSA?

BSA-Shirley used serial numbers 1000-9999 with single and double letter prefixes, and serial numbers 30000-39999 with single letter prefixes.

I would say serial number 'M1501' is a 1942 BSA. BSA used the M prefix again in 1944 but these used the 3XXXX serial numbers.