I have had the misfortune to lose the screw-on cap that covers the "Extension System Terminal" on my MkIII. Never actually used the terminal, but one day in the field the cover just went AWOL! My concern is that water could easily find its way into the terminal and cause an electrical problem.
My friendly Canon dealer has been unsuccessful in getting a replacement. Seems it is a "spare part" that isn't listed in the catalogue - no part number - so they can't get Canon Europe to ship it!
If anyone has access to such parts or knows a man who does, I'd be very pleased to hear from them!