You got a real good deal, unless the rust is much worse than you described.
The 28-2 revision was the first three screw started in 1961-62 time frame, and was produced until about 1982 when S&W dropped the pinned barrel & recessed cylinder. Both changes were introduced fairly gradually so there aren't real solid dates on when the changes were actually made for each model; plus the dropping of the P&R features seems to have had lots of extra parts used during the transition to produce guns that had pinned barrels but not recessed cylinders, and also recessed cylinder guns that didn't have pinned barrels.
The serial number should start with an "S" if the gun was made in 66-67, or an "N" for a production date in 75-77. Look at the bottom of the grip frame for the real serial number and not a production number meant to keep matched parts together during finishing.