A friend recently acquired an overhauled O-290 to install on a new to him J5. I’m embarrassed to say I’ve never encountered one in the wild, my previous Lycoming experience is O-320 and larger.

We noticed this lever on the forward right side of the case. Parts book calls it a hydraulic valve, anyone know what it is? Oil pressure valve is in the normal spot further aft on the right side.

I would guess it's for a 2 position propeller.

Didn't one of the old long obsolete props use that control?

That looks very similar to the prop control lever on a Continental C-145-2H or O-300B.