51-04-01 MARTIN: Applies to All Model 202 Aircraft.
Compliance required not later than April 1, 1951.
In order to eliminate a fire hazard due to the possibility of flow from the tank to the engine compartment in case of the failure of the carburetor vapor vent return line, install a swing type check valve at each fuel tank where the vapor vent line attaches to the tank so that flow can take place only toward the fuel tank.
(Martin Service Bulletin No. 138, dated November 30, 1950, covers this same subject.)