The "LK" in front of the rear sight.

It is the forge marking of the company or plant that did the original forging

The LK is for "FN / Kaiser" - probably your question - but I asked a similar question about this one below, and was informed mine was a Cerro mark.

Check additional information HERE - Biggerhammer.net:  upper markings

Yeah, I want to know what the LK means.  I couldn't find much about upper markings in the search option.  But, it says that FN / Kaiser makes it in the link you sent me.  TY

Quoted: Yeah, I want to know what the LK means.  I couldn't find much about upper markings in the search option.  But, it says that FN / Kaiser makes it in the link you sent me.  TY

I updated my response for you

found it at Biggerhammer.net

HTH

sorry.

I cant say I have seen an FN carry handle. Is yours mounted on an FN upper or FN weapon?

It's on a Bushmaster lower.  The barrel is stamped "B MP  5.56 NATO 1/9 H BAR" and this is on the front sight post.  It's an R with a crown.

To Copelc the LK mark with the  "R" crown on the FSB is probably Bushmaster assembled parts.Forge Im not sure but im almost 100% sure it came from Bushmaster.I have an FK,BK and keyhole for A2 uppers with the"R" crown on the FSB and a DK A1 with the same"R" crown on the FSB.I know the DK was a diemaco upper that came through Bushmaster.Also Bushmaster uppers are usually grey baked dry lube on the inside does yours have this as well?What is stamped in the top of the barrel past the front sight base?is it BMP 5.56 NATO 1/9 HBAR..if so then definately Bushmaster assembled.They make their own barrels and now machine their BFI marked lowers but everything else I beleive is subcontracted parts.