I’m not aware of the plates ever being mounted right side up and they don’t easily flip over because of the bolt holes. Some aftermarket ones may be different. In many cases they have just been removed.
It’s easy to test the valve positions, and install the hoses correctly. When under your EB looking at the valve from the rear, the center hose is the outlet. I believe the left one is the main tank and the right one is auxiliary. Trace a hose back to a tank to be sure which is which. Remove the center hose, remove the left hose and attach a short piece of hose you can blow thru. Then set the selector to point to the main tank as labeled on the indicator plate. Blow thru the hose and you should get air out the center outlet. If so, the left port is the main tank. Attach your hoses accordingly.
Just be aware they are probably old hoses and may crack or put up a fight. Sometimes it’s easier to figure things out and live with them but it sounds like you are pulling the auxiliary tank anyway so might as well sort it out right.