Reliance brand speed indicator tachometer could be used for early race car speedster or others, 3 1/2" face, nickel plated solid brass construction ( could be stripped to bare brass), porcelain face. Unusual 3600 RPM. Marked on back 1/2 to 1, that is the input ratio to the face marking, meaning that this is intended to be driven by the input shaft at 1/2 the speed indicated. On a typical engine, drive this is camshaft speed and it will indicate crankshaft speed.