In the late 1940s and early 1950s the US military was looking for an upgrade to the M1919 that could feed from either side for use as an improved coaxial machinegun. Saco-Lowell developed a model that had the driving spring attached to the back plate (eliminating the need for a mainspring and driving rod protruding out the back of the bolt), a solenoid trigger for remote firing, a feed cover that could open from either side, a bolt with dual tracks that could feed from either side, and a reversible belt feed pawl, ejector, and feed chute. The experimental T151 had a flat backplate (?), the T152 had spade grips and a "butterfly" trigger like the M2HB, and the T153 had a pistol grip and back-up trigger like the M1919A4 and an extended charging handle similar to those on the M1919A5. The T153 was adopted as the M37 and was produced by SACO-Lowell and Rock Island Arsenal from 1955 to 1957. It was in regular service from 1955 until it was replaced by the M37E1 in the late 1960s and the M73A1 in the early 1970s.