The Lyman 55RWS goes on a laundry list of bolt action .22 rifles.(R--Remington,,W--Winchester,,S--Savage).
Winchester 67 (but you had to alter the safety!),,various Remingtons including the 500 series and the Savage 23 series rifles. It also fits Marlins, H&R's and on and on.
The separate slide with peep disc is the other half of the sight.
The Armstrong was called a Dual Range Aperture or Dual Range Peep IIRC.
The allen wrench that fits in the site base is often missing.
They were around in the late 40's perhaps and in the 50's for sure. The company didn't last for very long.
No idea what it may fit, most likely a Mauser or Springfield action, but that's just a guess of course.
Some models of their sites used a 'v' block adapter on the base to allow mounting on various shape receivers.
The sight was a competitor of the Lyman 48 at the time.
They show up on auction sites from time to time. Vintage site collectors and those that like iron sighted rifles from that era like these as they are fine quality sights and something a bit different from the standard Lyman and Redfields more often seen.