All I can tell you is, when I bought my RRA 15 or so years ago(flat top w/ FF 16'' bbl) it came with a YHM rail with that type of barrel nut assembly. Unfortunately, the nobs red locktited the damn thing on there so in order to take it of to change the barrel I had to destroy the damn thing(and I tried everything including a torch!). Your hand guard didn't come with a nut I guess? It looks the same.