No, that exception for horizontals only applied to the ones that were manufactured prior to about 1998 (???) because there were no OPD valves available that could be retro’d into them. And,the exception only applied to the requirement that the valve had to be OPD. The tank itself still had to be in good shape, no dents or rust, etc.
Mine were always OPD, and all of the recertification rules apply to them that apply to vertical cylinders.
Also, the permanently mounted tanks are covered under different rules. Our removable cylinders are covered by DOT rules. The permanent tanks are covered by ASME rules.
:):)