I believe your rifle (assuming it came with the mounts and scope) is the USMC version of the M1C sniper rilfe. It had the Griffin and Howe mount (the FSN means federal stock number)
The rifle normaly came with the Kollmorgne Rifle Scope 4XD. The rifle, if in orginal shape would have either a cone or pronged flash hider. Also it would have to be a Korean War ventage as the Marines didnt adopt the M1C/Ds until 1951.
Most (Army and Marine) M1Cs and Ds came with the M84 telescope which was 2.2 Power and if I remember right was made by Lvman.
This is just my guess, if you can post a picture, we could probably tell you more about the rifle.
When I frist started running sniper schools for the National Guard, we used the M1C/D. You'd be suprise at the accuracy of these rifles in good hands.
We made consistant long range shots up to 1100 yards using M72 30 cal match ammo. Yeah people are gonna tell me thats way beyond the Max Effective Range, and it is, but I had some pretty good students who could pull it off.
We replaced the M1s with the M21, which in my opinion is probably the best sniper system out there. (That statement will catch some flac also--- So Be It)
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071