Kohler Shelfback Faucet stem and bonnet assembly. Stem is 3 3/8″ long overall, and spline count is 19. Bonnet nut is 1 1/8″ across the flat sides. These were commonly found in 1960’s vintage Kohler lavatory faucets. Made in the USA.
BE SURE to pay attention to “direction of turn” more than “temperature”. These stems can be tricky.