On the cover where the 2 upper tabs bolt on, there is a line between them, non-od units didn't have that. Non overdrive units were cast iron too. It also looks like you have the larger bearing retainer like the OD unit.
That line (rib) there was used in the '70 and later 4 speeds when they changed from the ball & dent side cover to the one with the levers. So thats not an overdrive specific part but all original overdrive trans do have it.