Dk4 distributor.

[John Mason]

Hello to all.  I have a March 1935 Ruby fitted with a Dk distributor all the while I have had it the small bolt that fits from the block into the groove on the outside of the dizzy has been missing and the car has run well just relying on the clamp to keep it in place.  However recently on retiming the engine with the clamp bolt loose I found that I could not turn it clockwise far before there was no more adjustment. After some head scratching I lifted the dizzy slightly and found I could adjust everything ok. I also realised the purpose of the bolt that goes into the groove on the body of the dizzy.
Can anyone tell me the size and thread of this bolt.

John Mason.