GeoJammy, I'm with you. When Ruger went to the GP-100 and SP101 they dropped the Service-Six/ Security-Six and Speed-Six line. There was some overlap in production of the GP-100 and the Six series from 1986 -1988 but none for the SP101 which was introduced in 1989.
Many Ruger fans, myself included, mourned the loss of the Six series revolvers but Bill Ruger had no intention of keeping both lines in production after the GP-100 came along.
The GP-100 is a super strong gun and one of the best DA revolver designs of all time, unfortunately it is also a bit large. I've always felt that the GP could stand to go on a diet.
You are not the first person to look at those two designs and wish for the newer locking system with the smaller contours of the older revolvers. If Ruger could incorporate the Peg style grip frame and non-rotating ejector rod into a gun with a slimmer, half lug barrel and a slightly smaller cylinder; they would have the best of both worlds.
The Match Champion comes close but the cylinder still has way more steel than it needs and the barrel could be round instead of slab sided.
While it is true that factory replacement parts for the Six series guns have all but dried up, those guns rarely break. Ruger also made about 1.5 million of them, so you could cannibalize one for parts if needed.
If you have the opportunity to purchase a 95% +, Six series revolver, I would suggest you grab it.