This is an attempt to provide a physical description of the Tap-O-Cap die. I apologize for the crude artwork, and I regret that I cannot provide more precise measurements with the equipment at my disposal. I am hoping that with a little luck, this might be good enough for a skilled machinist to fabricate a working copy of the Tap-O-Cap.
Note - all dimensions are in inches.
The Tap-O-Cap die consists of two major components - for lack of better terminology, I will refer to them as the cylinder (left) and the piston (right) -
The Cylinder is all-steel construction, closed at one end. A 0.157-inch diameter steel rod is mounted (apparently pressed) into the closed end.
A thin slot is cut into the side of the cylinder, immediately forward of the steel rod. This slot is cut most of the way through the cylinder's diameter. The next photo shows a paper card inserted into this slot.
The piston is all-steel construction. It consists of a hollow shaft with a knob screwed onto one end, and a set of teeth on the other end (vaguely resembling a hole saw).
At the base of the teeth, the inner diameter of the shaft is 0.31 inches. This tapers down to a 0.21-inch constriction, and then opens up again to 0.245 inner diameter.
There are 12 teeth, arranged in six pairs. The pairs are separated by pronounced notches. Evidently this arrangement enables the die to fold thin sheet aluminum into #11 caps - the skirts of the caps might be described as hexagonal in cross-section.