If anyone wants to know, the bezel of the speedometer screws on and off of the casing. I bruised my hands trying to loosen it, but it finally came off after prying at the case to try to get the threads loose.
I stripped off the odometer and trip odometer numbers so I could blast the rest clean with solvent. Its filthy with crud on the inside, and simply wouldn't work properly. The trip odometer went back together easily. The main odometer will go back together tonight. Its an interesting mechanism, not at all like a car speedometer. It actually samples the speed and holds it until the next sampling cog comes around.