Don't think this is "rare" but must surely be uncommon.

Used double action Rugers are plenty scarce around here.

This one is:

Low Back
Fixed Site Security Six
2.75 inch barrel (my favorite)
- and -
.38 Special

Nice gun!
I'm also a big fan of the Ruger "Six" series revolvers. And yes, they are getting harder and harder to find.
I'm no expert, but I do believe that with fixed sights and a square butt, that this would be a Service Six Model.
The .38 Special caliber is scarce and desirable to some collectors. The low back has some collector value, however now days its very difficult to find aftermarket grips to fit it.
But still, a serious Ruger collector would love that!

I found this 150 series 6 inch Security Six in March. I wanted larger grips and finally in September I found a set of Herretts. The early 150 series low back Security Six are hard to find aftermarket grips for.

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SWCA 1932 SWHF 135

Definitely of the hard to find realm. You may find this interesting, taught me a few things about the Ruger 150 series:

RugerForum.com • View topic - Ruger Security Six "lowback frame"

The caption version - a stainless lowback with an unusual and most likely not factory serial number. ATF arrest material, Ruger prototype or something else? Never did quite get to resolution on the mystery.

hittman77 is a serious Ruger collector, or, if he does not like to think of himself as one, he is at least seriously interested in Rugers. The earliest fixed sighted Six series revolvers were stamped Security Six. Since he called it a Security Six that most likely is what's stamped on it.

That would be quite a find for me. I was happy finding a much less interesting 4" .357 Service Six.

You see Ruger Security Six's around here , I won't say often , but you see them . The only thing I can figure is people brought them with them when they moved down here . They start to retire and put them up for sale . There's one area south of Tampa which is a well known retirement area for NYPD and NYFD officers . You see some older gems offered for sale over that way .

Very Nice. The Ruger Six series Double Actions are underrated.

Here's my .38 Spl Speed Six - It has the later High back

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Sceva
OGCA SWCA NRA

The Security Six with adjustable sights are plenty common, like these here.

The FIXED site Security Six was made before Ruger got into the LEO market and "invented" the Police Service Six. Ruger re-named the fixed site Security Six to Police Service Six and began putting adjustable sites on the "new" Security Six.