So I pulled the Final Draft script for Lee Cronin's "The Last Gunfight" (dated May 9, 1968). Scenes 54 through 58 have the relevant content:
54 ANGLE AT CITY LIMITS SIGN
In a line exactly coinciding with the city limits sign,
all four of our people walk firmly into an invisible,
but perfectly firm and impenetrable was [sic--"wall"] of force. They are stopped in their tracks... with considerable
grunting and bruised noses. They recover themselves,
feel along the invisible wall. It is endless.
LORL [sic--"KIRK']
Well, that answers that question.
Force field.
They look up as a rider approaches, from town, and
waves cheerfully at them.
RIDER
Hi, Ike. Hi, boys. Good luck
tonight.
The rider passes through the invisible was [sic--"wall'] and rides away. Our people stare after him, frustrated. Kirk
gets an idea. He looks around, spots a horse tied up
nearby. He crosses toward the horse. The others follow.
55 ANGLE ON HORSE
As Kirk approaches. He knows nothing about mounting
a horse.
McCoy
You don't know anything about
horses.
KIRK
Simple. Just put your foot in
this... and jump up.
He does. He now sits backwards on the horse, facing
the rear.
SCOTTY
I don't think that's the way
you're supposed to do it, Captain.
KIRK
Simple matter of orientation.
He starts to turn around.
SPOCK
Where are you going with that horse?
KIRK
Are you blind? Didn't you see
what that other horse did? What
one horse can do, another horse
can do. It's simple logic. I'll
send him back for you.
McCoy
As I recall, they hanged horse
thieves.
Kirk spurs his horse forward.
56 FULL SHOT - HORSE AND RIDER - STUNT
Yes, indeedy, the horse gets through. But the rider
is swept off with great force as he is even with the
city limits sign... knocked to the ground with great
force.
57 ANOTHER ANGLE
As Scotty, McCoy, Spock and Chekov dash up to their
downed Captain, who is having some difficulty getting
his wind back.
CHEKOV
Are you all right?
KIRK [sic--"SPOCK']
There was a flaw to your logic,
Captain.
KIRK
(gasping)
Yes, of course. I'm not a horse.
SPOCK
Precisely, Captain.
They help him rise. They hear a clock striking. They
react to:
58 ANGLE ON CLOCK
WHICH POPS OUT. It is 3:00. They look at each other
in dismay.