Mavic makes their own , plastic , split-slot , with a shoulder ,
it stays in the rim hole until removed with intention.

http://www.sjscycles.co.uk/mavic-rim...esta-prod3335/

And, Schwalbe presta tubes, have a shoulder in the ring nut , to center it
in the larger hole.

I used the aluminum ones I get from the LBS,
on my 349 , 16" wheel when the S/V tubes sell out at the Wholesale level, they still have P/V to ship..
so I adapted the rim...







....