The M-55 8-inch Self-Propelled Howitzer is the sister vehicle to the M53 155mm Self-Propelled Gun, differing almost entirely in the mounted gun and a few other details. Both were manufactured by Pacific Car and Foundry based on the M46, M47 and M48 Patton tanks. The M53 155mm gun tube is longer and thinner than the tube of the 8-inch gun mounted on the M55, the main way to visually distinguish the two models. M55 8-inch SP Howitzer was known as the T108 in prototype.
The M53 and M55 self-propelled artillery were employed by the U.S. Army and Marine Corps beginning in 1952. Most of the M53 155mm Self-Propelled Medium Guns were converted to M55 8-inch Self-Propelled Heavy Howitzers, starting in 1956. The requirement for air-transportable heavy artillery, powered by diesel engines, led to the replacement of these units with the introduciton of the M-107 175mm Self-propelled Howitzer and M-110 8in Self-propelled Howitzer in the early 1960s, gradually phasing out the M53 and M55.
Since the introduction of the M53 and M55 self-propelled artillery in 1952, there have been published Technical Manuals for the two models jointly. The original manuals were numbered in the TM 9-xxxx series, superseded in the late 1950s by the current numbering system of the form TM 9–2350–210-xxx. Examples include:
TM 9-2350-210-12 Operator's and Organizational Maintenance Manual (M53 and M55) (30 JUL 59 to Change 9 06 AUG 71)
TM 9-2350-210-20P Organizational Maintenance Repair Parts and Special Tool Lists (M53 and M55) (17 NOV 59 to Change 2 10 SEP 62)