back in the day, when S&H and others were working furiously to get as many HK sears made and registered before May 19, 1986 most of the cut down HK94 guns (MP5 conversions) had cut down 94 barrels which were threaded and "fake" three lugs were installed. In the late 80s and into the 90s, there were manufacturers and dealers recommending not to acquire suppressors that attached on those three lugs as they might work the fake three lug threads loose. Back then everyone thought swapping a HK barrel was a big expense and it was easier (cheaper) to turn down and thread the portion forward of the three lug and go with a threaded can. Back then there was a large variance between the precision and lockup of different manufacturers three lug cans vs whichever manufacturers fake three lug on the gun especially where being concentric to the barrel was the utmost concern. Lots of folks were happy to go threaded since it got the best concentricity back then before three lugs tolerances tightened up. Seems like today if a can manufacturer is splitting hairs over knurled protector vs smooth protector perhaps they should simply include one with every can out the door.