There were probably engineering prototypes. Who knows...
It was generally believed that the Industrial 1388032 was the first Model M, then we found out about this part number a while back, so I wouldn't be so quick to assume that because you haven't seen some ancient terminal keyboard from the mid 80s that it doesn't exist.
On the flipside, I was reading the web page of the guy who inverted the inverted-T arrow cluster, and he claimed that IBM's first inverted T using terminal keyboard was this one here.
The AT keyboards were manufactured up until 1987, as the Model M supplemented them as opposed to replacing them. There were still terminal Model Fs being made into the early 90s.