It's not a serial number, it's a casting number, and mine also has a / in the number...
The bolt pattern for the 3.1 is the same as the 4.2..
The bolt pattern for the 3.2 is narrower..
Two completely different engines sizes...
The cylinder for the 3.1 is manufactured by Kolbenschmidt..
The cylinder for the 3.2 and 4.2 is manufactured by Mahle..
Therefore, a muffler for a 4.2 will fit the 3.1
The 3.2 Sachs is lighter than the 3.1, so very few 3.1 engines were used for model airplanes..
They are a very good engine and run about the same...
Engine on the left is a 3.1
cylinder in center is a 3.2
cylinder on right is a 4.2
bolt hole pattern on engine is 1.5 inches
bolt hole pattern on 3.2 cylinder is narrower
bolt hole pattern on 4.2 is 1.5 inches
What the @#$%^& else can I say ?
My engine case is painted silver by Elliot, who converted Sachs engines...
Yours was either not painted or has had the silver paint removed...