My guess is that WGIII
stands for Writer's Guide revision 3 and TY
is for Tomorrow is Yesterday.
However, the info is a bit different in the Writer's Guide reveision 3, dated April 17, 1967:
THE U.S.S. ENTERPRISE
The U.S.S. Enterprise is a spaceship, official
designation "starship class"; somewhat larger
than a present-day naval cruiser, it is the
largest and most modern type vessel in the
Starfleet Service. It has a crew of 430 persons,
approximately one-third of then female.
The purpose of the U.S.S. Enterprise is to give
our audience a "home base", a familiar and com-
fortable counterpoint to the bizarre and unusual
things and places we see during our episodes.
Where possible we try to emphasize and play to
the size, complexity, and varied functions of
the Enterprise. This does not mean you must
always use the Enterprise or start every story
The "Saucer Section" of the vessel (at the top
of which is cur command bridge) is eleven decks
thick at the middle. The Engineering Section
(to which the two engine nacelles are attached)
is equally large and complex, contains at the
rear a hangar deck large enough to hangar a
whole fleet of today's jet liners. Turbo
elevators, which can run both vertically and
horizontally, interconnect every deck and
compartment of this huge vessel.
Included in addition to our bridge, sickbay,
Captain's cabin and other familiar standing
sets, are the widest possible variety of labs
and technical departments, computer rooms,
storage facilities, passenger accommodations,
and cargo facilities.