I finally found the original thread I was looking for; sorry it took so long. I have to agree with "flathead4rd" that you do NOT have a distributor with a vacuum brake. I think the boss is cast into the distributor body as a provision for a mechanical tach drive (which isn't there either). Here are the pictures I took of the Mallory for my '51 Olds that does have a vacuum, brake. I think you can see it is completely different from what you have. In my third picture, I think you can see the disc inside the distributor that the piston with the leather pad rides on.
As an aside, this is a very interesting distributor. Not only does it have the vacuum brake, but it also uses a special Mallory "FlashFire" coil that has two primary windings, but only one secondary winding. Each set of points acts independently to alternately energize each of the primary windings that induce an impulse in the common secondary winding. It is a "poor man's" dual coil that doesn't need the expensive specialized cap and rotor most require. In essence, it is like all dual coil distributors in that it is two independent 4 cylinder units, firing alternate cylinders, thus the need for two condensers.
Oh yeah, the last picture is the leather pad that I make for both Ford and Mallory distributors (they use the same one). Note how thick it is.
