Just at a rough guess I would think the pumps I have, which are mirror images, are for either Ford or Chevrolet application. Both manufacturers probably used the same pump suppliers.
CMP vehicles would not have had exclusive pumps and they would have been a carry-over going back 20 or more years for all sorts of applications. I bet an air pump was offered as an after market accessory for most trucks at the time.
I would think Phil is correct in assuming these particular pumps were not for continuous operation and designed for idle speed tyre filling only. Trucks operating air pumps continuously for air brake systems would certainly have much different gearing.
I have no idea, but I think there may be a physics equation giving the optimum speed of a particular pump design and that is why modern high capacity pumps are screw systems which maintain the flow in a continuous direction rather than pistons.
Lang
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