The head markings are the copyrights of the original bolt mfgs. AMK has bought up a bunch of copyrights over the years but they haven't been able to buy them all. So they only offer a few of the head marks.

If you really need a special head marking and are willing to pay for it you can talk to AMK about running bolts for you with the special head marking. It will cost a fortune if it is possible. The odds are that it won't even be possible since the copyright might not be available.