Strange marking. I have never seen this type of identification on an aircraft bolt that I recall. However, I believe there is a custom bolt/hardware manufacture called LFC inductries in Arlington TX; and, the "X" is somewhat common as a manufacturer's mark for a lot of bolt manufacturers - what it means is probably coded within the companies literature. It may be intended for aircraft use but if it is custom stuff - who knows and probably hard to dig up. I have some recollection that LFC provided some bad bolts for Hartzell for prop counter weights which is probably memorialized in the FAA's ADs or service bulletins.

No idea if that helps any with what should be correct; but it might be worthy of a call to Kitfox to see if they have some special bolt for that particular location.

I am going to be doing my condition inspection pretty soon and will certainly check to see what head markings are on the bolt in our plane.