Early models/versions were Allison, later RR Merlin under license to Packard? IIRC.
Packard version was better as RR used system similar to Honda have used where sizes were random.For example in order to select a con rod bearing you first had to measure the journal dia & rod ID in order to know what to order which meant a huge parts inventory was reqd for service in the field. When Packard did the drawings for production USA style they used current practice of known size changes like std-0.010"-0.020" undersize journals etc