Maybe it's from one of these?

http://rayco-mi.com/SelfPropelled.aspx


I actually have a rear-end out of a airplane-"tug"- came w/ the farm (in a barn) that my parents bought from an ex-pilot- I never really thought about it before now but it might actually make a good trike rearend (it's real narrow)- also, it has a long lever attached that shifts between two gears in the axle. Unfortunately I'm nowhere near smart enough to ever figure out how to adapt it.