92 Grand Voyager Windshield Wipers

Another engineer I'd like to punch in the nose, but that's for another thread:
1992 Plym Grand Voyager: this morning in a raging snowstorm the pass
side windshied wiper stopped working. It appeared as though (from the sound) the linkage came undone. I removed the cowling, etc and found that was just what happened. The assembly is the wiper, a thing that looks like a knob with a shaft on it, and the end of the shaft is knurled. There was also a spring washer resting under the whole mess in the cowling.
I pressed it all back together using two blocks of wood. The way it went together I didn't think it was ever coming apart again. It lasted all of 27 minutes.
What is supposed to attach to the bottom of the splined shaft? is there a screw or clip I'm missing, or is it perhaps just worn to the point where it doesn't hold anymore?

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