Hi Champ62, your receiver is certainly a nazi 98k. Byf code dated 1944, manufactured by Mauser AG Oberndorf. Yours is one of the 1434219 made that year.
Cant see much more in the pics, but looks like the bolt has at least been modified, and overall it has been modified heavily.
What are you planning to do with it? A restoration? or sportarize? A restoration would be a labor of love, or at the point that it is in now you could do a cool high caliber sportarized rifle. With it being in the state it is, a sporter hunting rifle would be the easiest thing to do, but a restoration is always a fun project. Just in this case will likely be expensive and time consuming.
Either way ya go hope you have fun with it. Projects are always fun. Have a good one.