Thank you for the quick responses.
The sights are indeed marked 300 and 600.
I suspect my uncle may have picked this up in India in '44 or '45. He was one of the Lido road crew. This could account for the absence of the markings. Then too, this serial number appears to have been produced after US entry into the war. In either case, it appears the rifle had not been stamped... there is no evidence I can find that it was machined off.
By the way, I am curious about the single screw appearing on the receiver near the "No 4 MKI*" marking. Was this used for a scope mount or is it simply there as a conversation starter?