Sigh. Ruger's naming convensions on the whole SA product line have gotten more and more stupid over the years.
PRE-1973 (also known as "three screw" or "old model" guns):
The "Blackhawk" was the mid-sized frame similar in heft to the Colt SAA or New Frontier. It was normally set up as a 357 and could handle 357 full-house loads. First shipped in 1955 with a "flat top" topstrap and a grip frame known as the "XR3" more or less identical in size to the Colt SAA. In the early 1960s the larger XR3-RED grip frame came along and the topstrap developed "ears" to protect the rear sight. This was the first "mid-frame" Ruger.
(Small-frame means Single Six.)
Experiments were done at Ruger on this frame in late '55 or early '56 to chamber this frame in 44Mag. It didn't work. So the upsized "SuperBlackhawk" (large-frame) was created.
POST-1973 ("New Model" or "two screw"):
The mid-frame was dropped, period. All centerfire SA revolvers by Ruger shipped on the large-frame converted to the two-screw transfer bar mechanism. (Exception: some 32Magnums were built on the small-frame.) So a 357Mag could be safely re-barreled to 44Magnum and have the original cylinder reamed bigger and the result was perfectly safe. Heat treats and metallurgy were identical across all calibers. This is also where the era of the 45LC+P came from - the guns were 44Mag-grade and could deliver equivelent power.
The term "Blackhawk" and "SuperBlackHawk" now referred only to minor differences in caliber, grip frame and hammer. The basic frames were identical and any Blackhawk could be upgraded to SBH spec or beyond.
Later the "Vaquero" was created as a fixed-sight version of the same large-frame, same strength.
All large-frame post-'73 guns can interchange cylinders, base pins, hammers, triggers, grip frames, etc.
2005: the mid-frame finally returns.
The New Vaquero and 50th Anniversary 357 Flattop are built on the same pre-'73 mid-frame as the original Blackhawk. First mistake (esp. on the New Vaq) was in the name - "Gunslinger" or whatever would have been WAY less confusing.
Second mistake with the New Vaq was in chambering it in 45LC. Gunsmiths had for years refused to chamber old model 357s in 45LC because doing so would have created a Ruger 45LC that couldn't take the monster loads. Which is exactly what you get now with a 45LC New Vaq.
Snide posts in other sub-forums have suggested that I have to be wrong about this because there's no way Ruger would create the potentially explosive confusion of creating a single weak specimen of 45LC gun among a sea of high-strength monsters. But they damned well did. See also Buffalo Bore's specific warning on this in reference to their high-powered 45LC+P ammo if you don't believe me:
http://www.buffalobore.com/ammunition/default.htm#45colt
Buffbore also sells lower-powered 45LC appropriate for SAA-class strength including the New Vaq - note the difference in horsepower:
http://www.buffalobore.com/ammunition/default.htm#low45
Ruger should have jumped into the 44Spl with both feet. Gunsmiths have been chambering old mid-frame 357s in this caliber successfully for years. There is also a lot of speculation and (apparantly) a few successful attempts at setting up the new mid-frames in 41Magnum...apparently there is enough cylinder wall beef. I am a bit leery of this mod, esp. if very stout loads nudging 44Mag power levels are involved. True, there's more cylinder wall "beef" but...hrm.
Anyways. This is the situation we have.
The good news is, the quality control on the new mid-frame series is excellent. They're some of the best and tightest wheelguns Ruger has ever made. They took the opportunity to reengineer the manufacturing process and put it to very good use. All mid-frame cylinders are bored with a single drill bit instead of six-at-once, so variances between cylinder bores (esp. in 45LC) are eliminated. All other machine tolerances are excellent. Ruger hasn't quite hit FA quality levels but they're ahead of where they were.
If you can live with the horsepower available in the mid-frame, highly recommended.
Parts interchange:
The new mid-frame guns can still swap hammers, triggers, pawls, most assorted springs and fiddly bits, grip frames and ejector housing parts with their larger post-'73 cousins. I am told (but cannot confirm) that the barrel threads are likewise compatible, so that's one obvious source for a 41Mag or 44 (for a 44Spl) barrel if you're interested.
Cylinders and base pins are (very) different.
I don't know if pre-'73 mid-frame cylinders will fit the new mid-frames. It would be interesting to find out - some shipped with both 9mm and 357 cylinders and the 9mm cylinders turn up on EBay once in a while...
I know the base pins won't swap with pre-'73 anything.