In July 1981, a report was submitted in close collaboration with the artillery system engineering working group (ASEWG) to develop a generation of concept self-propelled howitzers with anti-armor capabilities, along with all their various support systems. The M109A2, dubbed Class I, was used as a baseline from which these various concepts were drawn up and compared to. The main commonality between all concepts was that they used 155 mm howitzers.
Class II concepts were designed with the express purpose of defeating moving armored targets, surviving counterbattery threats, and providing fire on a timely basis. Concept IIA consisted of a casemate SPH. Concept IIB consisted of an all-new turreted SPH. And lastly, Concept IIC was a heavily modified M109 SPH using systems designed for the Class II concepts.
Concept IIA:
Concept IIB:
The objectives for Class III were identical for for Class II, and the same shoot-and-scoot philosophy was being heavily emphasized. Similarly, Concepts IIIA and IIIB were casemate and turreted SPHs, respectively. The major difference between the two classes was the ammunition being used.
Concept IIIA:
Source