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https://en.wikipedia.org/wiki/Heavy_Expanded_Mobility_Tactical_Truck#HEMTT_models
The M978 is a 9,500 liter capacity tanker. The current model is the M978A4; there was no M978A1. The M978A0 was produced in both potable water (approximately 18) and fuel servicing truck variants, the A2 and A4 models have only been produced in the fuel servicing truck variant