Post more pics, especially of the bolt serial numbers and stock markings. The Waffenamt inspection stamps on the right side of the receiver will tell you who made it. The stock takedown disc looks a little too big to be a JPS stock, JP Sauer used a 25mm diameter disc throughout the war as opposed to the 30mm used by everyone else during the war. A 1939 JPS K98k should have nearly every part numbered to the rifle so take lots of pics! There's another 1939 a block rifle in the picture reference thread for you to compare to.
Here is a nice example of an early JP Sauer 98K rifle. I bought it locally at the Jackson show (Craig, it looks like ya didn't get EVERYTHING up here). I don't want to upload all 70+ pictures I have of this, but if there are any parts that y'all want to see, let me know.
You may want to examine the stock more closely since the butt plate is mismatched, as well as the f marked lug. Is the bayonet lug matching? Stock should be numbered inside and out originally even if a period replacement.
Sauer did use limitedly 30mm disk, they show up in 1944, possibly other years.
Stock on the above gun has been cleaned hard or sanded. I would think too early for that recoil lug, also. If the buttplate has been changed, that stock would have to be carefully vetted if original to the receiver. When any of the stock metal is out of place, red flags go up.
I guess it all boils down to price on this, in my mind sub 1K.
Jeff