Below are some photos of screen used TOS velour (1st & 2nd season style) and nylon diamond weave (3rd season style) tunics; as well as a 1996 velour TOS-style tunic that was screen used in the celebrated Deep Space Nine episode "Trials and Tribble-ations" as well as the 2005 Star Trek: Enterprise 2 part episode "In a Mirror, Darkly". When viewed together, some differences in construction details and the use of materials are readily apparent.
The following three photos are of a TOS 3rd Season Command tunic, a TOS 3rd Season Engineering tunic, and a TOS 1st & 2nd Season Sciences dress. Note how the black fabric employed for the collar material differed between the 3rd and earlier seasons of TOS, as well as the obvious difference between the earlier velour fabric and the final season diamond weave material.
The slide mechanism that joins or separates the teeth and the teardrop shaped handle or "puller" of the zipper are a common component for all three seasons; with the teardrop shaped handle also being painted to blend in with the main costume color.
The photo below shows the slide and puller mechanism for the 3rd Season Command tunic; which was not visible in the picture above.
The 1966-69 zipper slider ...
The 1996 zipper slider ...
The following two photos illustrate both the braid and velour material differences between the 1966 blue and the 1996 red uniforms.