This is the early version of the 6.5, 7.5 series front housing for the 15mm crankshaft. Several versions were made of this. This has set screws to hold the carb/venturi. Some versions have a sprial in the of the crankshaft bore between the intake and front bearing. Some had a vacuum hole between the front bearing and intake. This has neither but should fit parts from any of the 15mm crankshaft housings. Can be used on outboard and aero. Carb neck diameter is .625"