On the Mk13 Mod5 the suppressor slides up on the barrel and the lock pins slide down into the two grooves on the barrel, the pin protruding at the bottom is a locator pin for the suppressor.
I just could not justify getting a KAC suppressor sent over here, so I went with a Swedish made Stalon Victor L suppressor, which has a somewhat similar slim profile of the KAC mk11. Heres my mod 5 (ish)