Here's a C5AF distributor, for 1965, with the revised mechanical advance. Note the "10L" marking, which indicates half the total number of degrees of timing. Expect to see 13L, 15L, and 18L as well. Only one side of the distributor cam provides the function, which means you can turn this cam 180 degrees to achieve the opposite number. So if you're on the 10L side, you get 20 degrees of advance. If you see 15L, you get 30 degrees of advance. And yes, at 18L, you get 36 degrees. Spring tension determines when mechanical advance begins to happen and how quickly it happens.