“Standard” Version III Tub Shower stem.
Sometime in the 1950’s, Standard redesigned the bonnet nut assembly for their tub-shower valves. Valves using this stem still would have had metal cross handles and “deco-ish” trim. Overall length is 5 5/8″ long, stem shank diameter is 7/16″, and barrel diameter is 7/8″. Be sure to compare your stem against this one pictured and the #86-8001. Cross handles usually use two hot stems, levers use one each of hot and cold. Made in the USA.
Handles are #80-0100
Escutcheons are most likely #80-6003