(I also posted this under "No pool cover.")
I'm all set to open my rapidly-greening pool for the first time by myself. I've figured everything out EXCEPT the two plugs pictured below, beneath the return jet eyeball fittings. They're the same size and material as the eyeball fittings, so I'm thinking that maybe they're used to close off one return for testing the pipes? My system is old and somewhat fragile, so I'm reluctant to fire up the pump without feeling confident that I've replaced everything correctly.
Does anyone recognize these, or have ideas? I promise not to blame anyone if their guess is wrong!