This one, the 1976 Chevy C-50/Firemaster, was originally a Cabin John engine "minipumper", converted around 1983? for the hazmat unit.Have photo of it after going OOS from FD (early 90's) with a large orange "cap" covering rear body, and the lettering; JB Marco 721-7308.
I would guess a welders rig.
I would guess a welders rig.