Managed to talk to Dad today. He was a mechanic and then service manager at a number of dealers in the Sydney area in the 60's and 70's. Cut his teeth on the factory servicing of EJ/EH's, whilst FE/FC/FB/EK/EJs were still returning for service.
He remembers using the mileage tester through both the 60's and 70's, though not for every tune-up. They were only used when a customer came with a complaint about poor mileage, and were used to verify that the car (either pre- or post-repair) was returning appropriate fuel consumption.
The process that Professor outlined is correct - the unit is connected between the carb and fuel pump, the rubber hoses run through a partially opened bonnet, and the test unit placed inside the car. The units had a clip on them that allowed them to be clipped onto the window, and then the window wound up (similar to a drive-in speaker). The car would be started and driven off, drawing fuel through the tester. When the "test track" was reached, the inlet valve was isolated, letting the carb draw only from the test unit (Dad had several "test tracks" through suburban western Sydney
). The starting odometer reading was taken. Once the unit was empty, the odometer was again noted, and the inlet valve opened (letting the car draw from the fuel pump). The car was driven back to the dealership, and the fuel mileage calculated.
Cheers,
Harv (appreciator of old Holden tools).