It's an 8mm hex. Mine wouldn't work, but it looks like someone used a metalic hammer to set it at one time, and the 8mm won't work anymore. Out comes the Dremmel to rescue the rear end.
Fred.
I was able to salvage mine with one or two small files and a lot of patience. The P O or the stealer probably never heard of plastic or aluminum hammers.

Or a steel or iron hammer and a block of wood.

Guess some folks just don't care iffin they get a might behind and wanna speed things up. Old mechanic told me a long time ago-if ya ain't got time to do it right-when ya gonna find time to do it over.

RIDE SAFE.