Hi,
if it is a single stalk indicator then the 'pin' could be a small lug that is screwed into the steering column that activates the cancel arms on the switch assembly.
If it is a 1980+ model then it should have two stalks, Indicators on one and Wipers on the other, and those assemblies are cancelled by two small roll pins that are fitted into to the base of the steering wheel that push on the indicator cancel arms.
The mechanic could also be talking about the long pins/rivets that hold the assemblies together or the brass spring loaded contact mechanism which could also look like 'pins' if one has broken off. Normally the plastic breaks and I can't think of any where pins have fallen out.
If you can post a photo of the assembly you have, single or twin stalk, then we can probably help with a photo or two to make sure we are talking about a genuine Moke item.
Terry