That bit about horizontal mounting is a holdover from the old days. A lot of the old-time vessel examiners in the Auxiliary and Power Squadron will still talk about it, and maybe even the folks they've trained. These are the same people who will invert and slap the extinguisher to see if the agent inside has clumped in the bottom.
My understanding is that both the agents, and the way they're processed have changed since the old days. Clumping is no longer a big problem. There are no references to it in today's vessel examiner manuals, or any other current source.
It's the same with Y valves. In the old days, the only use for a Y valve was to select whether the toilet pumped over the side, or into a tank. Nowadays many builders set the head up to always pump into the tank, and from there it's either macerated overboard or pumped out. But VEs and boarding officers still look for that Y valve. I've been tempted to install one that does nothing, and put a big padlock on it, just to save time explaining.
