Speed Six Versus Security Six
1)Speed Six has fixed sights only and Security Six has adjustable sights(Except for just a few of the early 150 S/N prefix Security Sixes which had fixed sights).
2)Speed Six has a slightly rounded grip frame(At the bottom rear of the grips)but the Security Six has a square butt frame.
3)Security Six(Except for a few "Rarities" or "Export" guns-and, some very early production guns)was only available in .357 magnum caliber. Speed six, can be had in .38 Special or .357 magnum(With a few possible exceptions-as Ruger made alot of special contract guns over the years)! An example, would be a few made in 9MM caliber(Or, possibly, .380 Rim?)!
4)Speed Six can be had in 2-3/4", and 4" barrel lengths(Commonly!)with a few(Model GS-33's)made in a 3" barrel length(Such as was made for the US Postal Service(And, a few of these guns made as an overrun!)! These three-inchers, also had a heavier barrel, than the normal 2-3/4" Speed Sixes! The Security Six could be had in 2-3/4", 4", and 6" barrel lengths. Some later production had these same barrel lengths, made in a heavier barrel profile!
5)Mechanically, both the Speed Six and the Security-Six are identical!
6)Both the Speed Six and the Security Six were made in both, blued or stainless steel!
7)Early(150 series, S/N prefix)Speed Six and Security-Six had a different grip frame than did later(151 S/N prefix, and up)Speed Sixes and Security Sixes! 150 series and 151 series "Stocks" will not interchange! 150 series stocks are very difficult to find, especially in non factory, custom stocks! 151 series "Stocks" are still made by the major grip manufacturers!
8)Although you didn't ask, there is also the Service Six(Or, Police Service Six)that is basically the same identical gun to the Speed Six, with fixed sights only, but with the same square butt as the Security Six. These were also made in the early(150 S/n prefix range)which again had the different grip frame(That was sloping-and, was more of a western style)just the same as the early(150 S/N prefix)square butt, Security-Sixes. Later production Service Sixes were made in the 151 S/N prefix(And up)ranges(The same as the Speed Sixes and the Security Sixes were)! The Service Six was made in the 2-3/4" and in the 4" barrel lengths only! These were commonly made in .38 Special or .357 magnum calibers(Some export guns made in 9MM-possibly other calibers(Such as .380 Rim)(By contract))!
9)I'm sure that I might have missed something-but, mostly would be one of the rarer varieties of these Six-Series revolvers! My memory is fading right now!