Engine identification

The engine was sold as a 250cc and the engine numbers might suggest that too. At least if one thinks /B stands for the cc.

Nothing seemed to add up and I could not figure out the meaning of the numbers and letters. When I measured the bore and stroke I came upp with approx 70x89. That would be 350cc I my book.

Trying to decode the serial number by the info on http://www.railtruck.org/jap_dating.htm I came up with the following:
I/S     I=350cc side valve, 70x90mm bore and stroke, S=1929
75163/B      75163=Serial number, B=Modified gear cover and exhaust valve lifter
H      H=Roller bearing pulley spindle
M     M=Crankcase without oil box.

Of the above everything seems OK until the letter M. At least there is something I would call an oil box.

But of cause the last paragraph on that page says "... this list is by no means infallible."