Given that New York-based Phil Tonken did the opening and closing sponsor billboards, I'm venturing this may've been aired nationally in certain markets - including, in New York itself, on WNBT Channel 4, at 10:45pm (ET).
Wonder how WGN got "Star Trek" in the first place - was it to keep after its NYC sister station WPIX Channel 11 that first acquired the show once it reached syndication in 1969 and continued running it well into the '90's (to the point where Channel 11 became so synonymous with that series that they ran the subsequent first-run syndicated "...Next Generation" and "...Deep Space Nine")? In any event, I seem to detect 'GN used 16mm prints, given how it looked and sounded; 'PIX by contrast ran the show off 35mm prints. But this does seem to be another case of how some syndicated shows didn't stay too long in one Chicago station, compared with New York (though "Star Trek's" trek seemed a bit more stable than, say, "I Love Lucy's").
As a side note, when WPIX first got "Star Trek" in '69, among other new syndicated shows they picked up was Phil Donahue's gabber, then based out of WLWD (now WDTN) Channel 2 in Dayton, OH - and running late at night. It was years at that point before he set up shop in Chicago at WGN. By then, two NYC stations - them, and WNBC (for a first go-round in 1971-72) - had very short-lived stints with the show. Not until Channel 4 picked up "Donahue" in 1977 for its other stay at that station (after another short-lived stint, on WOR Channel 9) did he finally catch on in the Big Apple.