Herter's 'XK3' was a Mauser 98 clone.
The earliest (early 60's) were made in W Germany for Herters. The actions should be stamped somewhere 'Made in ((W) Germany'. Sometimes the markings including serial number of the action are under the wood line on the assembled guns. This is pre-GCA so things were a little easier then.
All steel inclu the trigger guard and floor plate. Sliding side safety,no Mauser 'wing' safety on the shroud. Solid left wall.
An FN Browning type 98 ejector/bolt release on the left side held in place with a large headed screw. Not the classic Mauser style and it fails the looks test for many. But it's functional and more steamlined than the original design,,hows that for a marketing point.
Shortly after that, Herter's started having the action made in Yugoslavia by ZCZ. They are marked as such,,again sometimes under the wood line. The German made ones were probably too expensive,,or the Yugo made ones just too inexpensive to pass up for the mail order co. Also called the XK3,,also all steel.
The XK3 became the J9 still later. Still made by ZCZ in Yugoslavia. A few had narrow integrel scope rails, but I don't recall the mounts being offered.
Still a 98 Mauser clone and all steel as above.
All of these sold as actions, bbld actions and complete rifles.
Two action lengths available.
Some are nicely made and finished,,especially the German production. Some are rough machined and sloppy polished w/a soft wheel. That's usually found in the J9 production from Yugoslavia.
Judge each on it's own and as far as value,,no more than any other 98 Americanized Sporter would sell for. They aren't exactly rare, but a German made XK3 can be a nicely made action.
I think Herters had them bbl'd in the USA w/Douglas bbls,,at least early on. Markings and any bbl proofs of foreign origin can tell a different story.
The 'U9' was built on the BSA action as already pointed out.
I think they used the basic BSA 'Monarch' action for the Herters rifles,,but seems that there was something different about the Herters guns.
They should be well Brit proof marked on the action. Herter sporters again may show less than the careful workmanship of the BSA marked rifles. But any can be a good buy if you're not lable consious and don't go wild in bidding.