I have a model M 1958 engine. At idle my generator shows 12 volts output but at higher RPMs the output drops to 7 volts or less. Is this a generator problem or a voltage regulator issue?

I have an M series engine in my 1958 Chris Sea Skiff. I don't find any identifying information on the transmission so I'm not sure what reverse gear I have. I changed the oil in it this season and used straight 30 weight detergent BUT NOT Synthetic. The transmission worked fine for the first 7 or 8 ...

In the attached picture is the red arrow pointing to the oil fill?