Howdy
I looked for a Model 24 for years. They are scarce as hen's teeth. About ten years ago I ran across this Model 624. I changed out the Oversized Target grips that came on it for a set of standard Magna grips. A real nice shooter, but not a Model 24. There was a problem with the steel used in these, I think it was the steel used for the cylinder. I checked with S&W and mine is OK. That's what the deal is with 'the box with the red C'. I paid $625 for it at the time.
I kept looking for a Model 24. I found one of the more recent ones with the little bird's head grip under big rubber grips at one point.
A few years ago I found this 44 Hand Ejector, 4th Model Target, the precursor to the Model 24. Sorry, I don't have it handy right now how much I paid for it. This one shipped in 1955, two years before the name was changed to Model 24 in 1957. Once I found this one I stopped looking for a Model 24. I also got rid of the one with the little bird's head grip under the ugly rubber grips at that point.
Before 1957 it was known as the 44 HE 4th Model Target, introduced in 1950. (the Triple Lock was the 44 HE 1st Model, then there were three more versions as the years went by)
In 1956 the upper side plate screw was deleted, making it a Four Screw Smith. (yes, mine has the upper side plate screw, it is a Five Screw)
In 1957 the name was changed to Model 24.
24-1 in 1960 the extractor rod was changed from right hand thread to left hand thread, about the same time as this change was made across the board with all S&W revolvers.
24-2 in 1961 the cylinder stop was changed and the screw in front of the trigger guard eliminated, making it a Three Screw Smith.
The Model 24 was discontinued in 1967.
Reintroduced in 1983 as the Model 24-3. Pinned barrel deleted at this time.
4" barrel, 2,625 reported manufactured. Not really "rare", but still not a whole lot of them. (I'm quoting the Standard Catalog of Smith and Wesson, your seller probably is too)
6 1/2" barrel, 4,875 reported manufactured. 5" and 5 1/2" barrels are rare.
Model 24-3 production ceased in 1984 after about 7,500 made.
24-4 in 1990, New Yoke retention system for the "Through the Line" Special (no idea what that is)
24-5 Four screw frame, Performance Center Heritage Series Model. (pretty sure that one is no longer being made, at least I do not see it in their current line up.)
Regarding a slight turn line on the cylinder, ALL S&W double action revolvers will develop a turn line. Not like a Colt SAA where a turn line is a sign of abuse, with a S&W revolver it is part of the design. It is wearing the correct Oversized Target grips of that era.
Bottom line, if I did not already have my 44 HE 4th Model Target and came across that Model 24, I would be all over it like a cheap suit. I don't think $1200 is out of line. They are not a truly rare gun, the seller's data seems correct, but as I said, I looked for a Model 24 for years and never found one.