As my book is just the Second Edition, I have a hunch that my SN look-up isn't as well defined as the current Third Edition and some of these letter prefix SN's get hairy. Jim may arrive with a "more correct" date but I believe very late 1990 or early 1991 is the date.
As to +p, it has been S&W's standard line that if:
--the revolver has a stamped model number (1957 or later)
--if it is not specifically stated otherwise
--if it is SAAMI spec .38 Special+p
...then it is good to go AND endorsed by the manufacturer.
Two side notes...
Some folks and (worse!) a few small ammo manufacturers will occasionally ABUSE the term "+p." In the case of people, it is most often casual ignorance. In the case of BUFFALO BORE ammo, it is a wanton disregard for established industry standards and I find it to be an obnoxious act.
For the record, .38 Spl runs 17,000 psi max and .38 Spl+p runs 18,500 psi max, and I say that just to convey some idea of how much difference there is.
For reference, .357 Magnum runs 35,000 psi max.
Run .38 Spl+P in your 90's era Model 49 just as much as your shooting hands will allow.