I think the TN stock block is designed to use the TN folding stock only (which can count as a compliance part on a non-SBR), and not the HK folding stock. If I remember right, the hinge/pin on the TN folding stock is larger.
There are really only two people doing USC/UMP conversions right now, I used TN and am happy with the build quality, but the turn-around time was crazy (13 weeks) and Wade at TN doesn't have e-mail, just a phone number. Ed at HDPS is a bit easier to get in touch with.
TBOSTIC makes what looks to be a great conversion block for the SL8, but requires extensive cutting of the receiver; but gives a great G36 look to the finished product.
I dont think he make one for the USC, but I have been wrong about many things in the past. I think he might have made a catch? for the folding stock on the UMP/USC if I recall, to hold it in place when folded.
He is a member here you might try sending him a PM in case he doesn't see your posting.
Just to clear up a few things since i am looking at parts for a conversion. The Top Notch block doesn't allow the fit of the UMP stock? Is there a big difference between the TN US folding stock and the HK UMP stock?
Another difference between the TN and other setups is the lower rear pushpin setup. The TN setup clips the rear of the UMP lower and uses an externally accessible pushpin that allows the lower to swing away without removing the big screws holding the stock block onto the receiver first. I believe on the HPDS , like the TE , the pin holding the rear of the lower to the stock block is covered by the the receiver and you have to remove the rear screws to allow the lower to swing away. However, you don't have to clip the rear of the lower.
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