The WLA "gunsmith special" is a real pain. The rear trunnion in mine was not 90 degrees perpendicular with the pivot pin and the front trunnion was so far out of whack that the barrel had to be bent, REALLY bent, in order to pass the barrel through it.
WLA makes some really nice stuff but, this is NOT one of them.
For what I paid for it, I could have bought a 100% assmbled and tested PPSh41-SA from Allied Armanment.
Do yourself a favor and buy one that is 100% complete unless you like dealing with such expensive hassles!