I posted this in Gunsmithing also.

One of the men in my church showed me a Modeal 98 Mauser this evening. It is a very nice custom rifle which I'd place as being made before WWII. It was broght home after WWII by his uncle. It had light engraving on the receiver; a very nice sight island with two blades; and had the military style ears and thumb cut-out for stripper clips. On the left hand side, it had what appeared to be:

St.M.G.Lt (the "G" had lost its top)
N 14,6 gr.

This was the only marking I noted on the rifle. The stock was a very slim, sporter with snabel fore-end and small cheek piece. It had the lovely "squared" panels on both sides of the receiver and magazine box. The pistol grip was like the vintage Browning A5's.

The front sight was a blade on a long ramp. There was a small front sling swivel soldered to the barrel which I'd place at about 23". My friend tells me its is chambered in 8x57MM.

Anyone have an ideal about what I was looking at. My friend mentioned having it tapped for modern mounts. I suggested that he hold off until we find out if it is one of those old rifles you shouldn't mess with.

If you have any informed opinions, I'd love to hear from you. Ku-dude

The St.MG is for Stahl Mantel Geschloss ( steel jacketed bullet) and the N means "smokeless Powder". The 14.6g is the bullet weight. Before 1912 the bullet weight was over the StMG afterwards it was the other way.
There are multitudes of German proof marks. Look in the "Standard Dictonary of Proof Marks" by Wirnsberger. Its pretty well available. The German Gun Collectors Assn sells them too.