I was perplexed as well when I got to that point... I ended up conning my neighbor out of a wheel collar since I didn't want to make an order for a single stupid collar. (no hobby shop close to where I live)

The wheel lines up about where it should be with just one collar to hold it on (no need for a collar on the "inside" side of the wheel.) Though the whole assembly seems a bit flimsy (haven't broken it yet, but it sure feels like it could happen with a hard bump), so I think I'm going to put a whole different tailwheel assembly on when I get around to it.