As a result of spin-off series, [Star Trek] became known as [Star Trek: The Original Series]. The standard abbreviation is [TOS].
The date of 2264 for James T. Kirk assuming command of USS Enterprise NCC-1701 is conjectured by 'ST Chronology': "Date is conjecture. Assumes [TOS: Where No Man Has Gone Before] took place 13 months and 12 days into the mission, per one conjectural theory for stardates. (The episode was set on stardate 1312.) This system of determining stardates was not used in later episodes, but is at least useful for pegging the start of Kirk's mission in relation to that episode. This is also reasonably consistent with Captain Harriman's line in [Star Trek VII: Generations] that the Enterprise-B, launched in late 2293, was the first Enterprise in 30 years without Kirk in command." Therefore it is assumed that [TOS: Where No Man Has Gone Before], which was the second pilot then first episode of the series, took place in 2265. Details of the rest of the [TOS] era uniforms are here: '2265-2270 / [TOS] ERA'.
There are actually two versions of [Where No Man Has Gone Before]: the one aired as [TOS: Where No Man Has Gone Before] and the one submitted to NBC tv network as the second pilot for [TOS] that has not been aired.
In [TOS: Where No Man Has Gone Before], the Starfleet uniforms worn by personnel aboard USS Enterprise NCC-1701, now commanded by Captain James T. Kirk, are the same as those worn in the 2254 era (the year in which events of the first pilot, [The Cage], are set), but with a few minor differences.
In 2265, the standard Starfleet uniform comprises a coloured tunic (colour-coded to indicate department) with embroidered rank stripes for officers, a pair of unisex black trousers, and black boots. Officer ranks are denoted by embroidered stripes on the shirt cuffs. The ship's insignia is embroidered on the left breast of the shirt. The insignia depicts a free-form arrowhead pointing symbolically to the heavens, representing humanity's trek to the stars. Within the arrowhead is a different symbol depending on the department e.g. command and operations, or sciences or ship's services. Modifications to insignia result in the familiar gold background, black border and finished appearance, although still slightly smaller than those used in the rest of [TOS] (those later ones are shown in the drawing below). Continuity began as regards insignia and rank markings: Kirk, Spock and Lt. Alden's insignia have stars, while Lieutenant Commander Mitchell and the Chief Engineer Scott wear what would later become the Sciences emblem, and Dr Piper, Mr Sulu, Dr Dehner and Yeoman Smith wear the Ship's Services emblem.
NOTE 1: indicates command personnel.
NOTE 2: indicates sciences and medical divisions. It took a little while in [TOS] for the department colour-code and insignia to be clarified as regards specific members of the crew. NOTE 3: indicates engineering and ship's services including the security division.
NOTE 4: this insignia is not seen in [TOS: Where No Man Has Gone Before].
The colours have changed a bit since service in 2254: Mr Spock has changed shirt (an anomaly as it indicates a change of department) from blue to gold, but in later [TOS] will resume the blue shirt, for the Sciences division.
The shirts retain the shoulder zippers, while Mr Spock's retained the snap fasteners enabling the shirt to fall into two halves to make it easier for the actor Leonard Nimoy to take on and off and thus save wear and tear on his make-up. 'The Star Trek Compendium' says that the captain's hat is never seen again (I must have not noticed the hat). Female personnel continue to wear trousers; note the bell-bottoms, a fashion of the 1960's when the episode was produced.
back of uniform
Dr McCoy in medical top
The landing party jackets seen in 2254 do not appear after that until the field jacket of the 2270's. Henceforth, unless landing parties (or away teams, to include the later term) require specialist garments such as those worn in [#58 Blood Fever], personnel wear the standard uniform.
