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 machine gun.
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.

The M37 was used mostly on the M47 and M48 Patton medium tanks.
The M37F was a trial variant fitted with special sighting equipment.
The M37C was a variant without a sight bracket designed for use in aircraft armament (like the skid-mounted XM1/E1 helicopter armament subsystem).

The M37E1 was a M37 machine gun converted by Rock Island Arsenal and Springfield Armory to chamber the 7.62×51mm NATO cartridge and feed the M13 disintegrating belt.
They were designed for interim use until the M73 machine gun could be fielded.
The M37E1 was to be standardized as the M37A1 but development of the improved M73A1 precluded this.
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 machine gun. 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.
The M37 was used mostly on the M47 and M48 Patton medium tanks. The M37F was a trial variant fitted with special sighting equipment. The M37C was a variant without a sight bracket designed for use in aircraft armament (like the skid-mounted XM1/E1 helicopter armament subsystem).
The M37E1 was a M37 machine gun converted by Rock Island Arsenal and Springfield Armory to chamber the 7.62×51mm NATO cartridge and feed the M13 disintegrating belt. They were designed for interim use until the M73 machine gun could be fielded. The M37E1 was to be standardized as the M37A1 but development of the improved M73A1 precluded this.
M1919 Browning machine gun