Great Theoben find!
You are correct. The muzzlebreaks are venom. From what I remember they were officially called "executive" muzzlebreaks. I have one loose somewhere. Rare find, that is for sure. Not positive why there would be open sights on a Theoben.
Did own a countryman at one point. The front sight was a one piece cast, so there was a shadow through the scope. So I sold it. That was basically a 19ft lb R-9. If it wasn't for that front site I'd still own the gun. Probably worth a lot more now since Theoben was still in business when I sold it.