I can imagine a set-up where one injector (perhaps no bigger than your average five-gallon bucket) feeding six chambers located hexagonally around it, like the chamber of a revolver. The actual tubes may or may not actually revolve.
Actually, although my TOS Enterprise
deck plan thread has taken a temorary hiatus, a revolver mechanism with probes and/or torpedoes is exactly what I imagine in the stern part of the phaser control room
from "Balance of Terror" behind the grated window.
However, "to keep a number of [non-solid, energy ball] torpedoes at the ready, because arming the torpedoes is an operation that requires some time" is a concept which - IMHO - is too sophisticated and out of sync with the TOS context featured in the series.
"Obsession" featured a good example how they'd transport antimatter: in a solid pod
I think that's what a TOS photon torpedo warhead would have looked like, spearheading a missile that would have consisted of Nomad and other recycled parts.
And then there's the "Errand of Mercy" dialogue to be considered:
SPOCK: Minor, Captain. We were most fortunate. Blast damage in decks ten and eleven, minor buckling in the antimatter pods, casualties very light.
I think there is good reason to have antimatter storage "down there" for the photon torpedoes, but the way I see it these antimatter pods are either warheads for a photon torpedo or spheres that can be used as mines or the like.