1937 Russian Capture with grip questions

[elpato]

Hi,
I recently traded a used HK USP for this PO8. It's a 1937 dated S-42. The pistol is mostly matching, except for the sideplate, sear bar, extractor, and firing pin. It came with the black grips shown in the pictures, I believe they may be original, but not correct vintage. When I received it, it was missing a locking bolt spring, and the sideplate had been assembled improperly, with the tab on the rear on the outside of the frame. The former owner had apparently used a hammer to close the takedown lever, as the front tab on the sideplate was galled where it was forced closed, raising a ridge of metal on the tab, preventing it from closing. I took a file and worked the ridge down to where it would close properly. A fourm member (GT) , sold me a new locking bolt spring. I cold blued the file work on the tab. The pistol now functions like it should, and shoots well.
I had a few jams with a new Mec Gar magazine, and winchester white box, the ejected brass would end up with the base of the case inserted in the breech, locking up the forward movement of the breech block. (i.e. Backwards). It functioned with S&B 115gr fine.
I guess I have 2 questions:
Are the grips originals?(if they are, I would be happy to trade them for some wood ones, original or nice reproductions)
And, any idea what would cause the jams?
Thanks for any and all help!

[elpato]

The grips:

[alanint]

Your grips are unfortunately reproductions.