Roger, roger ... minerals in water seem to lock this valve tight. I also broke handle.
I was able to unscrew the antisiphon valve from the hull [3 screws] to provide a little more room to play with the hoses and was able to do it that way. If I remember right I also had to unscrew the top cap on the antisiphon to get a bit more room, but I may also have had to pull one hose completely off the antisiphon loop to get the hoses back on the new valve.

But on the photo with the red arrows there is a inside wood strip that apparently is glued to the plywood face and then vertical screws seem to lock it into the shelf, but that edge may be glued as well?

good luck.

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[Linked Image]