I'd say that the FMP receiver is worth more than a PTR receiver, but seeings how you can get an SAR-8 for roughly a grand, not much more.
The last hk21 kit I saw, which was a while ago though, was asking like $4500 on sturm.
Take that minus the barrel, shroud, pack, stock, etc, and you might arrive at ballpark estimate for the feed box.
The "E" box on hkparts.net is $2400, but I am sure that the "E" one is going to be a lot more.
In my humble opinion of course.
The box stamping, I have no clue.
If you are looking to sell, keep me in mind. :)