I suggest trying to disassemble the valve by unscrewing the bonnet where the stem exits. This is the usual construction. It's true, there is no knurling, no flats for a wrench, but if this part cannot be removed, how was the valve assembled? I would clamp the valve in a vise, apply some penetrating oil
, tap gently with a light hammer from each side, maybe apply a moderate amount of heat, and try unscrewing the bonnet.