Crank openers (operators) first appeared in 1955, and the Hehr style of opener lasted until 1965. It was patterned after casement window openers, but they had a weak link. The gears were cast from soft pot metal, and by turning the wrong way with the window closed, or opened against a resistance, they are easy to strip the teeth off the gears. Replacement is the only option, and generic replacements are available from RV dealers/suppliers, such as Vintage Trailer Supply. See this restoration topic for more info.