Crab distributor fine adjustment

[d m moore]

Question re crab dist on my 1942 Canadian army truck - '42 Mercury engine. After having pertronic electronic ignition fitted I needed to make some fine timing adjustment using the slotted hex screw on the top right hand side of the distributor (not the vacuum adv/ret adjustment). The bolt seemed very easy to turn and I can see no locking arrangement. Could vibration make this adjuster move? Second question - does a 5 degree (say) adjustment of this screw equal a 5 degree adjustment of the timing? i.e. a direct relationship, clockwise advance, anticlockwise retard?