What in HECK was that old overdrive called

Sorta remembering...the one I'm thinking about was in either Popular Hot Rodding/Car Craft back mid 70's to early 80's. Before Gear venders. Some older dude had made one for his Charger and you could exchange or buy the parts yourself with instructions. I have the magazine somewhere. I was interested for my Swinger 340 4spd at the time.