If there happens to be an oval metal tag on the starter, the color of the tag tells the tale. 6 volt units (starters and generators) had black paint on the tag. 12 volt units got red paint as the base color. I'l bet yours is a 6-volt unless somebody has gone to a lot of work, including changing the flywheel ring gear, to run a 12-volt starter. The 6 volt and 12 volt starter drive pinion gears have a different tooth shape, and the flywheel gear has to match the starter.
Jerry
