Fuel/Air ratio, measures right and left tank, rich or lean fuel. Type B-6A, order no. AC-33213, MFR’s part no. 13500, spec. no. 94-27933-B, MFR- Cambridge Instrument Co, Inc. "To be used only with analysis cell of the same MFR patent no. 2,237,538. 2-3/4" x 3-1/4". Several serial numbers to choose from.