Please believe, I'm not picking an argument here.
Erich, I do not doubt what you write. I was not even aware of the differences in the 150 and pre-150 Ruger frames.
Howsomever,
Nimitz87, at the very low prices CDNN charges for their replacement stocks, I'd be very tempted to order one or two sets and do a little tinkering. Note: Security Six, Speed Six, Service Six, and Service Six are all essentially the same frame - - Perhaps one or more of these is in square butt only, and/or round butt only.
If 'twere me, I'd order the wood RB stock, and be prepared to do a little carving with an X-Acto knife, or (careful) work with a Dremel grinder. If you take out too much, you can "bed" revolver stocks with Accra Glas or even old slow dry epoxy, just as you can with a rifle stock. Just be careful and put a thin coat of release agent (or carnuba wax, or even petroleum jelly) on the metal parts, lest you obtain an unsatisfactory
permanent installation.
If you get the rubber version, it is a little more difficult to grind out - - Tends to melt if your Dremel is set on too high a speed.
Good luck to you.
Johnny