Guys,
Very often with 18th century taps there is no "cutting slot". The threads are swaged, not cut, as with modern taps. If the screw plate is hardened, as in a file will not cut it, then it is indeed a die or screw plate. Also, in the period the screw plate is hand held and the screw blank or shank is the part held in the vise. I believe that Dietle is correct in his description.
Here are some other 18th c taps, some have a small slot and some do not. The slot collects the very fine "swarf" not really a "cutting slot" at all.
Here is another "smith made" screw plate, they come in all different configurations.
Jim