$152 is a a very good buy for a No4Mk2
, many of which never seen service .
The date must be '49 since production of the Mk2 didn't start until then at the Fazackerly factory.
Converted Mk1 rifles would be stamped Mk1/2
Converted Mk1* rifles would be stamped Mk1/3
The difference between the Mk1 and the Mk2 is the trigger assembly and the forestock receiver inlet.