Scott, you may have gotten the 1980 reference from the RW article by Buc Fitch, W2IPI, a member of the Dobbins Island AM Expedition in the Chesapeake Bay from a few years ago.
Here's his article.
http://www.rwonline.com/pages/s.0053/t.416.htmlThe "G" and "T" variants were only two of the
three versions Gates produced in roughly the time frame mentioned, and looks really like he was referring to ALL kilowatt boxes in the era, not just the Gates. He also constrained the discussion to "all tube" boxes, and as Bill points out the "H" is a hybrid.
According to what I remember from some factory literature, the "T" ran from 1957-62 when the "G" came out and it ran until 1969 when the "H" came out.
Notice the lettering is not chronogical. The literature was a service memo from the early 90s advising stations who owned these models that it had been 20 years since the last one was made, and urging them to consider other options because the factory was, as of that memo, discontinuing support.