Hello Marathon Man,
For what its worth, SCSW, 3rd. Ed., page 398, shows:

1966-1967 S262000 - S289999.

However,I have a model 29-2, S/N S2636XX, probably a little over a thousand less than your 28, which was shipped in April, 1965, so nothing is ever etched in stone with S&W.

I think you'd be pretty safe assuming a ship date of later 1965 or early 1966, but the only way you'll ever know for sure is with a factory letter.
Hope this helps,
teesur.