original water heater in 68 overlander

I am interested in testing out the water heater on my 1968 overlander. So far, with the exception of the scary furnace which I pulled, the rest of the appliances work - fridge, AC, etc, so I'm encouraged to give it a try. It's a Bowen G10TF, no obvious signs of rust or corrosion. I tried hooking up a regular garden water hose to what looks like the intake, but the threads won't seem to grab, so I'm stumped (the threads look fine, is it just some other kind of hookup I need?)



How do I go about testing this thing? I need basics, I am not very handy with this kind of stuff. Thanks, anyone!