Paul:  Thanks, I do appreciate that comment.  Though this thread is mostly written just to keep everyone posted on progress with the Carawagon, I hope the content may also be of use to people working on the same operations in future.

Sunny Jim:  From my experience, you're correct in assuming that changing down causes more wear.  The overdrive in my 88" was at the point of requiring double de-clutching to down-shift, whereas it was still smooth changing up.  It was worn to the extent that I chose to replace the entire synchromesh hub when rebuilding it; the teeth on the 'down' side of the hub were rounded off.  It still isn't perfect now, as the synchro teeth on the output gear shaft itself are a little worn, however this component is no longer available new.

This overdrive, from the Carawagon, actually shows very few signs of use; I don't think it's done many miles.  It shifted smoothly in both directions when tested in the 88", so I've put things back as they were (barring new detent springs).  The only issue was bearing noise.  I wonder if that was simply down to corrosion while it was sat idle for thirteen years.

Phil