[Gustav]

Yes exactly right.The back plate with 'extended bearing thingy'.
Plus you need to make a long diff outdrive but thats no biggy.

I actually realised that sitting in front me is a vertigo diff locker which would look suspiciuosly like an 'extended bearing thingy' once it's been through the lathe to make the diameter 10mm all the way along the shaft and had threads put on after where the bearing sits.I'll probably make one from scratch but the locker could be used.I've figured out how to make strobe mount on a 10mm shaft(10x15mm bearing or bushing, strobe bored out to 15mm.And pads but they're already 14mm.)and fit in the length of the locker including plates, pads, strobe,narrowed sprong and big(but thin) nut.Whole assembly only c.1" longer than a stock diff.End of extended thingy sits on the usual 10x16mm bearing.

The flat back of the extended bearing thingy will be one plate,the other will be locked to it with a notch in it and short pin sitting in a groove running along the extenended thingy.obviously the pin can't go through the extended thingy because that would lock the shaft inside it.

Anyway hope that's clear as mud but as you say metalman it's actually rather simple:C: .