Seems Allan used both methods to join the firing pin shaft and cocking piece. On my Hall-S, there are two 1/16th" hex heads, one to secure the firing pin and the other on top of it to lock the first. My Hall-M has a roll pin. This fooled me for a while. It is set pretty deep and there was some oily dirt that hid its identity. The hex key, seemed to "turn" but nothing moved; it was just churning the gunk. Uncertain, I finally broke down and called Allan Hall. I resisted doing this, because I knew he would give me a hard time about "messing" with his action. He did, but finally explained that the "M" almost certainly had a roll pin. It is important to make a "guide" to locate the depth the pin is set, because it is half-way into the firing pin shaft and half-way in the cocking piece. Driving it out fully does not happen, at least in my "M," rather the pin is driven completely into the firing pin shaft which then can be unscrewed from the cocking piece.
The "stop" on the firing pin shaft has a "dimple" marked to locate the position to turn the shaft when re-assembling it.
By the way, the "B" marking on the trigger hanger DOES stand for "back." Mine had been assembled in front (not by Hall). It did seem to function fine that way, but evidently there is some small dimensional difference.