With that casting # 3917124NF it is a 68 or early 69 rear. The assembly date of CI 1219B idicates that finaly assembly was on Dec 19th(1219) at the Buffalo plant (B). The previx CI indicates that it came from. the factory as 3:73 Posi. The 68-9 rears are 1 inch wider than the 67; but the 68-9 will bolt into the 64-67. There is a little difference in the pinion angle as the ears are not located in exactly the same place on the carrier housing. One easy way to tell the wider rears (68-72) from the narrower rears (65-67) is to check the distance between the backing plate and the ouside of the lower control arm bracket, if a cigarette pack just fits in the space it's a narrow rear, if there is about 1/2" wider than the cigarette pack it's the wider rear.
The carrier housing should also have a date code cast into the housing. Something like B 12 7 for Feb 12, 1967 etc.