1931 Chevrolet Ignition Timing

In the 1931 Chevrolet owners manual it says that the, "semi-automatic advance", activates at approximately 14 MPH. What they are referring to is the mechanical (centrifugal) advance consisting of the two weights inside the distributor.
The actual advance that is "added" by these weights is as follows:

RPM	Degrees Advance
350	0
900	11.5
1300	15.5

So if you set the engine to the "recommended" 18 degrees, then at full running speed (ie: 1300 RPM), the ignition will be operating on 18 + 15.5 = 33.5 degrees advance