Yeah I have 650b x 42 tyres, there is lots of space in the horizontal part of the dropout, which suggests I do need it. Which also probably means I should look harder on the shed floor for that spring or try to find a new one. Blergh.
I'll take a look in my 'random washers and stuff' jar. I vaguely recall seeing a couple of springs. If I find one that's the right size I can chuck it in the post.
Biro?
I use them to effectively block off the horizontal section of the dropout, so the axle just slots in vertically. I may well be doing it wrong though!