OK, if I remember correctly from old blueprints, the base of the neck along the spine of the secondary hull used to be where the deuterium tanks were.
I'm not sure if any old blueprints even acknowledged the concept of deuterium tanks. The TOS ship was originally supposed to run on something futuristic, be it superdense pellets of weird matter or interstellar forcefields of plain holy spirit (no doubt from Scotty's distillation plant).
However, the base of the neck (or much of the neck itself, for that matter) would be where a deuterium tank would go if the blueprints were made following the TNG era guidelines. The Encyclopedia
gives a deck count for the TOS ship where this sort of tank placement is suggested, FWIW.