77921 should be a mid to late 1960s gun and the barrel would be correct for the frame.

Serial #___ Date of Mfg.
104XX_______ 1958
149XX_______ 1959
189XX_______ 1959
295XX_______ 1960
1120XX______ 1969

The lightweight field barrel is 5" and has a narrower profile and different rear sight. Your barrel is a 5.5" Heavy barrel. Nice gun.