Glad to be of help.
According to the 'book,' N178xxx corresponds to roughly 1972-1974, but the best way - some would say the only
way - to date your gun is to request a factory letter. This service is available from S&W, but at some cost.
Serial Numbers vs. Assembly Numbers
Regarding the S/N-Assy. No. mixup at your dealer, this has happened several times during transactions I have witnessed; it's a pretty common mistake. Most dealers cheerfully accept being gently corrected on this, but I have seen exceptions (in which case, I simply defer to their 'authority').
As a general rule, the S/N always appears on the bottom of the grip frame of S&W revolvers. There are exceptions, of course, but those few are usually considered 'rare' collector's items.
Owner's manuals are freely available on the S&W website, but most (if not all...) are not model-specific. For example, they pretty much have just one 'manual' for all revolvers.
Original manuals can occasionally be found at GunBroker, various dealers, etc. It might take some looking, but they usually don't cost very much.
Example: SMITH & WESSON MODEL 58 MANUAL on eBay (Buy It Now is only $18 shipped...)