I've just found out the hard way that the shimano style freewheels don't fit on the ellipse. I put one on without realising it hadn't engaged the threads all the way to the end, first ride, my chain came off and the freewheel twisted and knackered the threads, ouch.
yeah, not too bad in my case as I only ride SS so as long as I have one good side it's all good. Cheers for the link, I'll get one ordered.