Hmmm, I might well not need both axles for my use, but honestly I don't fancy trying to explain that to a friendly policeman!
@Chris - this is a pre-1992 model trailer with taper bearings, rather than the sealed bearings yours has that require silly amounts of torque on the hub nut. The advantage of that is they're easier for user servicing (e.g. you don't need a bearing press). I suppose the disadvantage is that they need regular maintenance such as greasing, but I'm not really sure. Perhaps they stopped using them because they kept failing!?
I managed to find out that "the thing" is a "spirol pin", part number F1909S. It's quite literally a spiral spring pin, and I'm pretty sure it's "just" there to stop the hub washer from rotating and hence spinning the castle nut off.
This trailer was cheap because it had lost a wheel and cracked a hub, and I think that was actually *because* it sheared off the spirol pin. That will then have allowed the bearing to rotate the washer, the washer to put a torque on the castle nut, and the castle nut to fall off when the split pin also failed.
Overall, this trailer is a perfect example of what happens if you run a trailer for years and never maintain it. I can fix everything up and will enjoy learning in the process, but being honest, I do wonder if I should have bought a better one to begin with. Let's see how things pan out!