I found this website page to be helpful and thought perhaps others may too.
This 11 page document was published by BMC on May 1, 1964. It is a “decoder” in that it explains how to derive the type of part, thread as well as finish.
If you have the BMC Service Parts (AKD3227), it may also help to ensure you are using the correct fastener.
Author: A. Henderson
These engines used on 1962 and 1963 cars did not have a PCV Valve. Crankcase pressure was vented through a road draft tube from the front tappet cover, and through this simple rubber hose connecting the valve cover to a port on the front air filter backing plate. Vaccuum created by the carburetors pulling air into the engine pulled vapour from the valve cover/crankcase. The air filter backing plate for this application is unavailable, as is the valve cover itself.
Author: A. Henderson Pics: M. Feig
We believe this switch was used until May of 66/car 90394 when it was changed to a smaller section switch more like what was used in North American cars of the same period. With the arrival in NA of the Mk II cars, the dipswitch went to the turn signal switch.
The switch pictured below is stamped 4/63.