Welcome to the Team!
The over drive three speed was offered as an option way back when. The tranny was made by Borg Warner, but for a six cylinder, or small V8. Don't know the torque rating off hand but if it is an old late forties, or fifties era electric three speed I would consider replacing it with a modern five speed.
Now another way of getting an OD with a Saginaw three on the tree transmission would be with the addition of an aftermarket Hone-Drive or more modern Gear Vendor auxiliary transmission that fits behind your manual and is operated by the floor lever like a gear splitting two speed rear end in many antique gas powered semi trucks of the early sixties. They are indestructible and are rated at over a thousand horsepower. The Borg-Warner OD three speed looks like a modern four speed as opposed to the early threes speeds with out an over drive that had a long tail shaft with a tiny gear box.
Either way your transmission is unique.
An electric power seat is also rare. Most power seats of this vintage were vacuum powered. So yours may have been added out of a newer car.