PVX: I was a master control operator in the 1990s and 2000s, and the ISCI codes were 4 letters and 4 numbers. An Ad Council public service announcement started with the letter C and three letters for the campaign, the digits would start with the year the spot was created, the revision or version number, and the last 2 digits being the length of the ad, so a 30 second Smokey the Bear Forest Fire Prevention PSA from 1999 would have been CNFF-9130.
Additional note: CHCH in Hamilton began rerunning the series as of January 21, 2019, with special guest appearances by the Diff'rent Strokes main cast in that first episode (originally aired August 24, 1979 on NBC), and a larger cast than what we'd be accustomed to in later seasons as the girls grew up over the years.