is the number in my haynes book for the fiver found in the 123 280s. if that is the same as yours i can give some basic info.
it is the fiver that is essentially a four speed with cast iron case with an aluminum od case bolted on the back.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.