1/4" Series (Aro Series) Type "B". Brass. Plated steel couplers have hardened steel balls and stainless steel springs. Coupler plugs are heat treated to reduce wear on critical mating surfaces. The Aro Series of plugs are easily identified by the distinct appearance of the plug. The couplers are identified with one wide groove on the sleeve. Couplers interchange with Aro 210 series as well as units produced by other manufacturers. Maximum working pressure.