Specta, it looks like you have one of those old pre-OPD horizontals. Notice how the valve is sitting dead center in the tank end, and the valve handle is not the 3-corner style. There must be an internal dip tube that curves up into the vapor area of the cylinder. For that reason, there won’t be an OPD style valve available for that one. You may still be able to replace the valve with the same style and continue using the cylinder if it’s in good shape. I’m not sure what the rules say about revalving an older horizontal.
On the newer cylinder, the valve is offset from center, so it’s in the vapor area.
:):)