I agreee that the "magneto" switch means a spark ignition engine, and that magnetos are mainly small engines (up to motorcycle sizes) or quite large (airplane based?) or stationary engines (where they didn't want to worry about battery maintenance). Although magnetos were found on a few cars, most engines based on automotive designs were the battery based systems. The size of the panel suggests that it was installed in a fairly large vehicle, it is quite a bit larger than the panel used in most M series trucks. The transmission temperature and pressure guages seem unusual for a normal roadgoing truck of the period which suggests to me that either the transmission was marginal for the size of the vehicle or planned for heavy work off road.
The last picture includes the character string GM-G-136-C2. If they are referring to the vehicle type G-136, that is the M8 Greyhound armoured car.
This seems to fit with the idea that the 3 lines are serial number, vehicle type and identification of the accepting inspector for the ordnance corps (suggested by the bomb symbol). This last paragraph is speculation but seems to make sense to me.
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