I agree, this is a rare discovery, - slightly diminished by the probable sanded stock, but never the less an incredible discovery!
Late production, this is the last block, but they are known to the 4500/b block for sure, plus reported reliably to the 6000/b block. Very rare to find the original stock, unit marked and in good condition. One has to wonder what this looked like before it was sanded, probably as fine as the metal... While valuable, had the stock been untouched it would have been worth an incredible sum.
Better pictures would be helpful, the barrel code obviously, typically when original they are Imperial-Berlin style, in front of the rearsight, but very few have been recorded in such detail. The right receiver acceptance, the right side of the stock, typically Imperial cypher (Prussian) over C/O, but depending on history can be a little more busy. Of course and potential ordnance work would be good to see, but I expect little in this case.
Perhaps 4-5 are roughly this original, perhaps a few more, but this is a rare rifle. I have recorded at least 32 rifles (few anywhere this nice!) plus a few more reports, plus about a dozen hybrids, Oberspree/19's and some WOK/Oberspree's that have early S28 features and are serialed in the S28 sequence.
Try to do more pictures if you can!