For what it is worth it is my understanding that S&W switched from the "S" prefix to the "N" prefix in the early 1970's.
I'm pretty sure it was in 1968, when the GCA of 1968 was passed.
Ask the seller to remove the grips the serial number should be written on the right grip. If you don't see a serial number don't worry, about the same time, early 70's, S&W stopped matching the grips to the revolvers.
Well now that depends. There were two main grip types for the model 27 in those years, target grips (they cover up the butt and front strap, but reveal back strap) and magna grips (magnas reveal the entire frame, including the butt, so you can see the SN). Only magna grips had the SN inside the grip, and I think that actually continued to the late 1970s. Target grips at that time never had the SN inside them because the wrap around target grip required much less precise hand fitting whereas the magna grip required hand fitting so the grip would be flush with the grip straps of the frame.
Also, as for the target hammer, target trigger, and target grips, those were optional I believe before 1975, but after 1975, they were standard. Also the standard front sight was a vertical patridge sight (90 degree angle with barrel) but you could have ordered the gun with a ramp front sight aka baughman. So if the gun is say a N75000 gun, its prior to 1975 IIRC, and if it had TH, TT, TG and a ramp front sight, then we could call it a "loaded" or well optioned example. Standard equipment would be the plain hammer (not sure how wide, but much less than .500 which was width of the target hammer) and the trigger would be the same width throughout, whereas a target trigger flairs out from the top of it (where it enters the mechanism) then down.
No serial # on grips. Asking $700
Its well worth that IMO. The big thing about the gun is that it has one of the more popular barrel lengths. People like them in 3.5 in, 5 in and also 4 in. Many people feel that 5in is the perfect barrel length and Skeeter Skelton was famous for saying that he felt the perfect revolver was a 27 (or perhaps S&W 357) with a 5in barrel. As long as it has about 95% of its finish, I would buy the gun for $700.
And one other thing, IMO this is one of the best 357 DA revolvers ever made and it is at least in the convo with the Python. Of course maybe the Python had a little more hand fitting, but for all that hand fitting, the Python was not any better. The 27 had a fine polish, fine fit and finish, great sights, a great trigger and a great feel. It is also a very durable 357, if someone wants to make powerful reloads. Of course, one should never go above and beyond the specs in a reloading manual for a 357 ctg.