It is improbably, but not impossible that the overdrive is original to the car. Most of the 1970 cars wre low spec because the 140 was out and the 120 weas kept cheaper.

The overdrive ratio was lower in the overdrive cars, effectively you ended up wit 5 ratios where a non overdrive had 4, the revs ar cruising ended up about the same. If you're converting you're better off keeping the non overdive axle as it is more suitable for modern cruising speeds.