Advice I got from Terry Dyer years ago, was some work and others are a little soft and wear, so try it and see how it works. Not like it is nuclear science (and even if was Pakistani has built there own ;-), However, if you run across a German K carrier for cheep (ha ha, well actually I have ;-) or even a nice rewelded carrier, then would not pass on it, but for the time being would run with the one you have.
Can't be sure if someone sent it back and got the barrel changed by HK or one of the good HK smiths put in in. However, HK did not add paddle mag releases and if you had it done before sending the gun in, they would not change barrels, the last time I talked to them. If the paddle mag release has not been spot painted, then we can assume that the entire gun was repainted (nicely ;-) at one time.
My MP5K-PDW started life as new in the box (the box is still in the garage) that I put together by flash light on the tail gate of Toyota #1 (running Toyota #2 nearly 20 years and half million miles later ... so much for being a GM buyer in the old days ....) Friday evening at KCR so I could shoot the PDW during the Bumps rush at the lower range. Was the 90's and Shattered still had sufficient 8mm, but that was way too long ago.