4-Speed Identification

FomocoReformed said
How can you tell it's an OD side cover?

and I took a magnet to it, magnet shows that it's aluminum

I did buy this from a transmission rebuilder so it's still up in the air what's actually inside it, I'm going to open it up to be sure it's an OD. What should I look for?

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.