Just discovered that they are a perfect fit for the threaded end of brake light switches. Nice tidy protection for spare switch.

You've got too much time on your hands. Get the bike out and scare the old ladies on Balham high street!

Dai wrote: You've got too much time on your hands. Get the bike out and scare the old ladies on Balham high street!

 Actually had a useful reason to find out that may be worth sharing. The rear brake light switch failed and was seized into the footplate socket. I drilled it out with 7.5mm but that left a brass residual   blocking the threads. Couldn’t find an 8mm bolt with the right pitch until I tried the dust cap and found it fitted the switch perfectly. Dremmeled a groove into an old valve and worked it through the hole to clean the threads. New switch installed without fuss.

Now I can keep the air from leaking out of my switches. (Or smoke if they're Lucas switches.)