That is a pre 1961 third member probally. I saw one like that years ago, 1964 to 1966, in a 1957 Plymouth Savoy flathead six cylinder car with 3.73 in it that had been flipped up on one side so someone could rob the tranny out of it behind a bowling alley in Yucca Valley, CA back then. I should have came back later after work and grabbed that rear end for future use
Hindsight is always better than forsight, correct
I put a set of 3.73 gears in a "741" case years ago to try in my old pump gas street Duster, that stupid car ran the same ET and MPH with 4.10, 3.91 and those 3.73