The Security Six's frame was machined steel like any of the S&W's or Colts of that time.
Ruger changed to cast frames with the introduction of the GP series.
Unlike most other handguns, the frame, crane, hammer, trigger, trigger guard and several smaller parts of the Security-Six are produced from chrome-molybdenum steel investment castings. The barrel is a machined forging and the cylinder, which rotates to the left, is machined from bar stock.
I think the heft of the later gun is due to the design change from forgings to castings...but, that is just a thought...nothing concrete to base it on. I have a 1975 produced 2 3/4 inch six that is heavier than a 4-inch GP100.....The later guns are quite a bit heavier, too. I have a stainless 2 3/4" Security Six made in '77 and a blued 4" Security Six made in '73 or '74 that still used the 150 series frame. Empty, the snub is several ounces heavier than the 4" version. The extra heft would certainly help with recoil.