Can anyone recommend a good low cost gauge system to help id screw thread types, lengths etc so people can confidently buy various bits of fastening hardware for pinballs? At the moment it is very much hit and miss for me.

You could use a piece of plate steel with drilled holes for the bolts to just fit.
All the USA manufacturers use UN threads, #4, #6, #8, #10, so you only need four drilled holes.
Be aware though that Australian or European manufactured machines may have metric threads.
For example Gottlieb machines made under licence by Vifico in Spain will have metric threads