What in HECK was that old overdrive called

Are you talking about the old Chrysler manual overdrive that came on the mid 50's vintage three speed cars? It was vacuum and electric actuated with a parking brake type handle under the dash that you would pull out if you wanted to keep the overdrive from actuating. It would drop engine rpm by approximately 33% when actuated. I’m currently looking for one for my 53 Cranbrook 2DR. They are hard to find and worth their weight in gold.