It can't be a T-4 if it's a 1980, they weren't available until the latter half of 1983. It'll be a T-176 (most easily identified by the straight shifter and aluminum housing, a SR-4 (curved shifter and aluminum housing), or the elusive T-18a (straight shifter and cast iron housing). Before anyone says they weren't available in 1980, some were, and I have seen them mated to the D300 from the factory. I was most humbled by seeing it, because I said the same thing you're thinking, but I have heard they are rare.