Can you show us the speed change mechanism Rob? It looks as if the input shaft drives the shaft with the smaller gears and the step-down in shaft diameter, and that shaft goes through the end of the housing and can be pulled and pushed axially, or perhaps it is spring-loaded so it only has to be pushed? It is evident that the sliding gear is the one on that shaft.
I like the O ring seal right around the housing - that is a similar concept to the way Honda seals car engine sumps. Pity they don't use it on mower engine crankcase covers as well.
