Robuta

https://www.georgebark.com/product-page/1-4-28-jam-nut
1/4-28 Jam Nuts are mainly used on pedal rods to secure the Pedal Clevis & Rod from loosening. Sold Individually.
jam nutrhsteelgeorgebark
https://omnifasteners.com/product/jam-nut/
A jam nut is a thin, low-profile nut typically used alongside another nut to lock it in place.
jam nutomnifastenersservicescompany