One nevertheless wonders about FJ's torpedo placement. Surely he would have had some idea of the fact that the ship always spat fire from the underside of her saucer and seldom was seen from other angles at all.
Was this a simple goof, of being mistaken about the placement? Or a simple goof of assuming that the placement in the episodes was ambiguous and subject to artistic license, rather than set in stone
Just thought I'd add a little data to this and let everyone reach their own conclusions (it isn't like anyone would listen to me anyways

).
September 21, 1973:
"FJ begins work on the blueprints of the Enterprise, a.k.a. the Booklet of General Ship's Plans." (
source)
September 22, 1973:
TAS Episode
"One of Our Planets Is Missing" first airs and included this weapons effect.
Thanks for that,
Shaw, it's a good point. However, in the interest of nit-pickery

, I feel compelled to post a couple of additional items: one which weakens your post and one which strengthens it...
The bad news is that in that TAS ep, Scotty says, "we can cut a piece of the antimatter villi with the tractor beams and transport it aboard like that." So, at least according to the dialog, that'd seem to be (a) tractor beam(s) shown "firing" (though it
does seem awfully phaser-like in its effect):
The good news is that while there's no mention of photon torpedoes in the April 17, 1967 revision of
The Star Trek Guide, there
is mention of an upper saucer weapons mount
and the description of the "phaser" includes a "proximity explosion/depth charge" reference (presumably as seen in "Balance of Terror") where the "phasers" acted more like a photon torpedos:
(Despite this saying "The main weaponry," which implies secondary weaponry, there's never a mention of any other weapon system. If anyone has an earlier or later revision of the
Guide, I'd love to know what it said.)
So, it sort of seems that the real-world explanation might have been that the "do everything" phaser was later split into two distict weapon systems: the "squirts" of phasers and the "blips" of photon torpedos after the show was underway (the first "photon torpedo" mention seems to be in 19. "Arena", aired Jan 19, 1967).
But the intended split into lower and upper saucer placement seems not to have been actually realized by the various optical effects outfits, and money was tight, so they went with what they already had: namely, both "squirts" and "blips" coming from below the saucer . While the red SFX overlays in "Arena" are new, the underlying ship angles are the same as in previous weapons shots. I freely admit that's a "real-world/budget" explanation rather than an in-universe one, but that's my best guess as to what might have actually happened.