As for me, at age 6 I'd seen some episodes near the end of the third season as Star Trek completed its run on NBC in 1969.
From 1969 to 1975, syndication was sporadic in that the channel I first remember starting to watch TOS on regularly was channel 13, KCOP in Los Angeles. But I remember that they did some striped syndication for about a month and then stopped. They would occasionally start up again but they would never do a complete run of all 79 episodes (at least that's what I recall as I grew up through elementary school and went into junior high school.)
In 1975, channel 5 KTLA started running Star Trek on weekends (an episode on Saturday and again on Sunday at the same times.)
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That's when I started to catch and watch TOS on a regular basis; and as I recall they went in production order and not broadcast order; so I was immediately enthralled by the first two episodes I watched which were "Where No Man Has Gone Before", and the episode that remains my favorite Star Trek episode to this day (across all of the Star Trek series from TOS, TNG...etc. all the way through Star Trek Discovery and Star Trek Picard); and that's " The Corbomite Maneuver".
That's also when I fell in love with TOS's first season, and while yes there were a lot of good episodes in season 2, and a small few in season 3; I honestly felt TOS lost a lot after its first season in how it played out episodes.
To the question of which one was the last "new" (for me) episode. It was S2 "The Doomsday Machine". Like others I had found out the number and the names of all the episodes, and in 1977, just before Star Wars hit the scene, That's when I finally caught "The Doomsday Machine" and realized I had seen every single Star Trek episode made.
Like others I didn't realize that they edited things out of the episode and for syndication to make room for more commercial time. The first time I realized this is when I took a trip to San Francisco in 1978; and caught a rerun of the TOS S3 episode, "The Enterprise Incident" - and it had the scene where the Romulan Commander is speaking to the Enterprise and talking about it following the Romulan flagship to a Romulan port etc; but it had a part that I had never seen were after Mr Scott states he will blow up the enterprise and take as many of the Romulans as he can with him, The Romulan Commander states:
"You humans make a brave noise..."
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That small section was not in the version that I saw being aired on channel 5 KTLA.
So yeah that's when I realized that they were cutting little snippets out of the episode to make room for more commercials, and I became interested in what I may have been missing.
The first time I saw uncut TOS episodes was during the time the Sci-fi channel, (which is now known as the SyFy channel) began airing their Star Trek special editions; which expanded each episode to 90 minutes which included the uncut version of the episode plus a bunch of interviews with cast members interspersed in between during commercial breaks.
I actually disliked that because it made the actual episodes hard to watch because there was so much stuff in between the actual episode.
So like others I didn't really get to see every single episode completely uncut until I picked up the original yellow, red, and blue clamshell DVDs In the early 2000s; and then I went through each and every season in production order and over a one-month period after work finally saw all 79 episodes (plus the restored original pilot episode of "The Cage"; including the extra footage they had in case they were going to turn it into a B-movie style feature film release, if they didn't sell the pilot.)
And once I saw the unedited episodes, I couldn't watch the series on actual television anymore because I felt the cuts they made really hurt the majority of the episodes I really liked (like for example TOS S2 "Amok Time", Where they started to cut out part of the dialogue between T'pau and Spock where Spock is telling T'Pau that he will do what he must, but not with Kirk.
I remember watching it and syndication one time, and the line from T'pau where she says:
"Thee has prided thyself on the Vulcan heritage. Art thee Vulcan or art thee Human?!" (And her voice was dripping with disgust when she asked that final bit.)
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That an editor could decide that bit of dialogue is something that could be cut to fit 15 seconds of commercial time just really pissed me off; and I could understand why a fan of TNG who had never seen TOS before it had gotten to this heavily edited state, would think TOS isn't all that great by comparison. A lot of times the edits really hurt the episode in which they occurred.
That said, there's one terrestrial channel that I believe has stations in a lot of markets that still broadcasts TOS episodes completely uncut, and still in a 60-minute time frame; and that's HeroesandIcons TV. I know for a fact it's uncut because rarely do the commercial breaks go more than a minute, and I've seen the DVDs enough to know that nothing's missing.
A few years ago BBC America also ran TOS uncut, and they did that by airing a block of episodes where each episode ran one hour and 15 minutes with commercials. They also ran the HD versions so I was a little miffed when they finally stopped and went back to just airing TNG and VOY, although now they've dropped VOY and pretty much air TNG and DS9.
But yeah, if you have younger fans who are interested, try to expose them to a few completely uncut TOS episodes and see if their opinion changes (It probably won't, but you never know.)
