Not saying it didn't happen, but the 2 '68 4 speeds I had (340 GTSs) did not have reverse lights. 1 was Inland console and 1 was Hurst non-console. Both were originals near as I could tell. Possibly a mid-year addition?
You have a point. Not an "end all" indication, but the first year a reverse light is mentioned in the Dealership data books is '69. From '69 up to '72 at least all the books include mention of a reverse light for the 4 speed cars. '67 and '68 just say they have a "reverse lockout"