My Savage #4MKI 1941 sn 0C2804 has that same marking. The highest serial number for a 1941 dated Savage that has turned up in my survey on another forum. It is an sniper that was never fitted with a scope. No S51 on the buttstock but does have the serial number on the buttstock tenon. It only has the Englandicon stamp on the receiver ring and no importers marks. Picked it up a number of years ago and thought to be from Springfield Sporters. Sadly the original forearm is long gone but I have restored it with a new Savage marked one from Springfield Sporters. It has been fitted with a SARCO mount and old Weaver K4. Excellent shooter. I had to hone the bore of the SARCO mount as it was very rough and undersize.

My Savage #4MKI* 1942 sn 15C5798 does not have this mark. It, too, is sniper never fitted with a scope and is in original state. Will put up pics at a later date.