HBO's 40th anniversary is next year (the premium cable channel launched November 8th, 1972) and a search is underway for lost sports programming, particularly those programs broadcast between 1972 and 1975. Here is a brief post from the Smithsonian.com Reel Culture blog with information about the search, including a timeline of missing two-inch reels, from HBO Sports curator Max Segal.
According to Segal, 3/4 inch screeners were regularly made for these early programs, upwards of 20 copies per program, and finding these tapes may be more likely than the two-inch tapes, which may have been reused. In fact, 3/4 inch tapes of two basketball games have been recovered already; Segal also reveals two examples of people who had 3/4 inch tapes for years but unfortunately discarded them.
I realize that as a cable channel HBO does not strictly fall under the purview of Television Obscurities, but the search for missing television transcends the broadcast/cable divide. Any updates will be posted as I come across them.