I believe that you are correct as far as the 68 that you found on the ring gear.
As an example, my original, numbers matching, '66 Chevelle, has the following information stamped on the OD of the ring gear:
GM 3790628 1137 65
GM 3790628 is the Ring Gear P/N, Stamped on OD of Gear
1137 is the ratio; Ring Gear 37 teeth, Pinion Gear 11 Teeth = 3.36:1 rear end
65 is the year of Manufacture (1965)
As in your example where 68 = 1968; on the above 65 = 1965
I have the protect-o-plate on this car and the rear end was assembled in Oct 27, 1965 although my Chevelle was built in Flint in mid March 1966. One of those fiew examples previously documented in this forum where a rear end, for whatever reason, was not used immediately after assembly.