The original series definitely did not establish the photon torpedos to be projectiles with a casing and I really like what Andrew Probert said on the issue:
"I envisioned them as what we saw during the TV era, they were glowing globs of plasma or some sort of energy. They weren't giant capsules. I envision them as big, glowy, dangerous blobs of... scariness."
(Question # 6 http://www.trekplace.com/ap2005int01.html
While "Errand of Mercy" could
suggest that the photon torpedos did contain antimatter (Spock: "Blast damage in decks ten and eleven, minor buckling in the antimatter pods") the actual dialogue could also refer to "buckling" of the antimatter pods in the warp drive nacelles.
After all, Kirk asked Scotty in "Obsession" to get him some antimatter from the nacelles, not from a photon torpedo (!).
Theoretically, the "old" photon torpedoes may have simply used energy plasma from the matter-antimatter reaction (opposite to using this plasma to "charge" a phaser bank) but by the time the Making of Star Trek
was published they had obviously settled for the matter-antimatter torpedo concept.