As header. Stainless S5 mod M18x1 thread. Is a .30cal model, but stamped proof for .223 Rem and is on my ticket as a .223 mod.
I will post to RFD for £5 extra. Or collect from Sussex for £140.
Don't let the thead put you off if your rifle is a different one. Jacksons will change them over for about £40 inc postage.