Elgin convertible models were designed to be assembled in either hunter case, or open face design. It was hoped to save money by allowing the company to produce only one set of parts, instead of two versions of each watch. But this innovation proved too complex to make and service, and was deemed not cost effective.
Here in this image the bottom plate, dial side, of the Elgin grade 100 convertible. The difference between the hunter and open face configurations is that the stem is 180 degrees from the seconds dial (the forth wheel) in an open face watch, but 90 degrees from the seconds in a hunter case movement.
This plate allows the winding arbor to be in either of two positions, 90 degrees different from each other.
In this image we can see a linkage of two similar parts connecting both stem positions to the "snap" mechanism for the crown to snap in and out. Among the many unique features of this movement, the "snap" is here in the movement, rather than provided by a sleeve spring in the neck of the case.
The "snap" is provided by the business in the upper left. The hunter position for the winding arbor is directly pointing left. The open face position is facing down in this image. The jewel for the lower forth wheel, which is the seconds hand, is at the upper most in the image.
This movement has its ratchet in between, inside, a two part barrel bridge. The trick to converting from hunter to open face is that a main wheel, which turns the ratchet wheel, which winds the mainspring, can be positioned at one side of the other inside the bridge - a 90 degree difference.
This watch is being assembled in open face mode. The stem is facing down in these images. The unused void for the hunter configuration is to the right.
Adding to its uniqueness, the winding mechanism is on the top, or back side, of the movement, opposite the dial.
When in setting mode, a pin pushes up from the other side and lefts an arm, causing a main wheel to move up, off of the ratchet wheel, and engage a wheel in the center.
The cannon pinion on the other side rides directly on the arbor of this exposed center setting wheel, thus turning the hands.