Just google Herters U9 rifle and you'll find some info on the rifle. Basically Herters imported the rifle from England....rifle is made by BSA (Birmingham small arms) BSA made the rifle from around 66 to 74 and also sold it under their Name.
Own a 6mm Remington Herters U9 myself that I found in a local pawn shop a couple of years ago. It had been well used and not well taken care of. Must have spent several hours cleaning its bore. Pleased to find the bore was still in good shape. Trigger pull on mine leaves a lot to be desired, lots of creep. Some previous owner did a bad job of trying to hone the trigger sear too. But, rifle is fairly accurate, dispite the trigger. 1" or less groups at 100 yds....have had 1/2" to 3/4" groups.
If top of the receiver is machined for a scope mount and their are no rings with the rifle, you may have a problem finding the rings to fit the grooves.....kinda unusual size. Mine came with rings, but were too low for scope I wanted to use. Lucky for me, a fellow range member saw my rifle at range and I told him my problem with rings......week or so later he'd dug through his collection of parts from years of shooting and actually had the rings I needed. Just viewed a pic of a U9 Herters and noticed on the rifle, someone had reworked and drilled and tapped the BSA machined mount and installed weaver scope mounts on top of it.
Also, to remove the bolt from receiver: open bolt and pull it back a little, then push forward on the trigger, while pulling back the bolt. Should come out.