Having paid more attention to 'Wireless for the Warrior', I note that this set was the standard generator in the Low Power version of the Bedford QL Radio Radio Truck, and the alternative specified for the High Power version.
Also specified in the Canadian version.
Obviously this example pre-dates this use, but a lot more must have been aquired rhan I originally thought. Strange how rare they appear to be but Coventry Climax Generators replaced most Onan soon after the war and 12 volt generators have limited civilian use.
The High Power version normally carried the larger Onan.
Chris - I commented on the Petter being obsolete as you said the number 3 was listed as such in your EMER.