This is an amazing suppressor adapter for the 55x series. If you have a 55x with a one piece barrel (i.e. integral flash hider), it is worth looking at this adapter. I can personally attest that this adapter works with at least some of the JDI imported 550s and 553 LBs. There is a lot variation in flash hiders on these firearms. The Swiss Army STG 90 flash hider does not have internal threading on the end for a blank firing adapter. B&T Switzerland calls the same flash hider with internal threading the "550 Export", which is what a lot of the imported firearms from JDI have for a flash hider. The "550 Export" flash hider is 22mm in diamter and 54mm from end of the muzzle to where it begins to taper. If you have a flash hider that matches this description, this adapter should work for you. It is my understanding that this adapter can also fit a true STG 90 flash hider if a piece is pressed into it to make up the slight difference in length between the STG 90 and "550 Export" flash hiders.