I've misplaced the Nut that secures the revolving wheel/cable at the side of the transmission. I can not remember - Is there anything special about it? looks like it's M10/1.50 - any problem with just using a Nyloc nut? How about adding a washer, or perhaps a nylon washer?

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Yes you can use a 10 x1.5 nyloc. Dont know if you have enough room for a washer. The original is a thin nut.

Maybe this

https://belmetric.com/thin-fuji-nut-class-4...u=NFUJI10JAMCLZ

I have the other items on hand, I'll use them for the time being.. and grab a couple of your suggested one on my next Belmetric order, I like it - thanks!

Or maybe use a regular (thin if you can find one) with a drop of Locktite to get you through

Its a split lock, not a nyloc originally and they are unique. I have a spare re-plated original if you are interested.