Hi !
George Emanuel sent me an Email about your motor.
Your motor is a Swedish motor made at the SBM factories in Uppsala, Sweden.
It is Model 3,9 and Type 031 (the first numbers in the #number).
It was sold under labels Archimedes, Volvo Penta, Crescent and Monark.
Your motor is missing the cowling so what is was sold as is hard to say.
Depending of the picture on the carburetor (8/15,5/170) I can say it is produced in 1974.
Hoping this helps a little.
