With a large screwdriver having a good handle to grip and a blade that fit the slot snugly, and a lot of force, the screw finally came loose. It popped loose with a crack, like it had been glued. It came out easily with fingers after that.
I had difficulty getting both the knob and the handwheel off, as well. The knob has a set screw opposite the latch. With this loosened, even removed, the knob would rotate partially and not slip off more than about 1/8 inch from where it normally is. After a couple of hours on and off, twisting the knob, it began coming off a little at a time. The handwheel, too, posed this problem at the end of the shaft, but it finally came off, like the knob.
The reason for the problem was evident with the wheel and knob off. There is a cut in the shaft that is tapered. The set screw, which is pointed, is driven into this cut in the shaft to prevent rotation. The screw in the shaft prevents the knob from coming off. The edges of this cut are ragged to the feel. There were burrs at the end of the shaft, too. Age and wear have deformed the shaft along this cut, worse at the end. It needs to be carefully restored to roundness. I'm mulling over some ways to do that, that won't make things worse. Any thoughts from anyone?
(WR11-Shaft)
[ATTACH=CONFIG]520536[/ATTACH]
Below is a picture of the stop motion mechanism parts. In order, the large washer goes on first between the handwheel and the machine, then the handwheel, then the knob. The set screw and the shaft screw would complete the re-assembly.
(WR11-HandwheelParts)
[ATTACH=CONFIG]520537[/ATTACH]
I have some fairly good pictures of the inside of the knob, too, with views of the spring from two angles. (greywuuf) I can take more if you need or get larger format versions to you if you need to see anything up close. The little lever can be moved one way which is stable, having the latch retracted for free motion of the wheel. Flipping the lever off of the stable point, makes it spring outward, rubbing on the inside of the wheel until it catches in the slot in the wheel. Up to almost a full rotation of the wheel could be required, for it to catch. The stable point may be seen here as a step in the latch, a little above the top edge of the spring. The latch is shown in the off-stable or spring loaded position. The gentle slope of the lever against the spring gives the lever its springy feel, up to the step, where it is then stable.
(WR11-Knob1)
[ATTACH=CONFIG]520538[/ATTACH]
(WR11-Knob2)
[ATTACH=CONFIG]520539[/ATTACH]
Thanks for the help.
John