Distributor ???

Can any one tell me what the numbers on the SB vacuum advance canister arm mean.
See attached pics.
I'm rebuilding a couple of distributors, and am curios. These numbers are not visible when the distributor is assembled.
I'm geussing they are something to do with vacuum.
Dave

View attachment 1715618639

View attachment 1715618640


8.5 distributor or 17 crank degrees of added advance.