I decided to measure the Rim Lock Bolt, Rim Lock Nut just because no one else has. You can get rim lock caps plastic or rubber to go in the holes if you choose to remove them for wheel balancing for higher speed road traveling.
First of all a handy PDF table so you can easily lookup and covert 64ths of an inch to the metric equivalent.
64UnitConversionTable.zip
Rim Lock Bolt, thread width.
Rim Lock Nut, flat to flat.
It's hard to read at an angle so here is a close up of the calipers for use with the PDF table.
Just for sanity check, its a hair under 36/64ths. PDF says 36.0 64ths of an inch is 14.2875 mm. I used a 14mm metric wrench and it fit the nut as the tool needed to be used.