They would have cleaned well if the wash water were not lukewarm. Start one of these. Let it go through the pre-fill warmup spray and fill and start washing. Unless it is a gravity drain model where you lose the wash water if you interrupt the cycle, open the door and use an instant read thermometer to see what the water temperature is. It will be around 115F, if that, and slightly above that if you supplied 160F water to the machine as most people who had a dishwasher back then were instructed to supply to the machines by setting their water heater thermostats to that, which you could do with old water heaters. If it is a gravity drain, check the temperature of various items in the load at the end of the wash period with an infra-red thermometer. The wash was short; I think 5 minutes in the 12s. It was OK for fresh soil, but don't expect it to clean food that has sat and dried.  One of the reasons that people with these machines rinsed the dishes before loading them was to add some heat to the dishes to help prevent the wash water from losing so much heat to warm up the dishes. These are things we do not consider now with dishwashers with heating elements, but they were what you lived with back then.

None of these machines from this time period were really great, but Hobart was particularly crippled by their experience with institutional dish washing with machines with booster heaters for the wash tanks and the overflow of the 180F final rinse water back into the rinse tank to keep it hot and from there to the wash tank in multi-tank machines and the machines like the UM-4 that heated up and stayed hot by long term use washing load after load during meal service hours. Home machines did not usually get that type of use and it was not until the early 60s with the introduction of the 14 series that a detergent dispenser allowed a pre-rinse or pre-wash before the main wash, but already many machines had those two features.