This is interesting. I am pretty sure that most 816 tractors had Onan twin cylinder engines, but without the T designation (like the one pictured above). The 816T is a bit of a rarity. The 817 was also an Onan twin, without a T.The designation of T was used interchangeably by Gravely. On the 8xx-T or 8xxx-T tractors it was the designation for the tractor being equipped with an Onan engine. When T was used in conjunction with a K before it, such as on the 8xxx-KT tractors, it then designated a twin cylinder engine.
I thought the KT was specifically for a "Kohler twin"... hence KT.
**EDIT** I think I just misunderstood you. You are saying that Gravely was not consistent, but when they did use the T, it was for an Onan twin. And the KT was a non-Onan twin (in other words, a Kohler twin).