No, it's a factory overdrive unit that was available from 55-65 in both cars and light duty trucks. It is exactly your transmission with the overdrive unit replacing the standard tail housing. A newer version was available from 66-70 which was a full syncro Saganaw unit. Almost all manufacturers offered overdrive transmissions in that era. They were available in Studebaker's in the mid 40's and Ford's from 49 up. Packard, Rambler, Hudson, Kaiser, Nash, Plymouth and others all had their versions of BW overdrive transmissions. GM was late to the party having introduced them with the redesign of cars & trucks from closed drivelines to open in 1955. They are 30% OD.
Fred
