There is a lock washer. Gwyn Lewis has a fitting guide. Torque to spec, drive for a bit then re torque and hammer the lock washer into place.

I have said this before but back in the day I recon Land Rover had Man Mountain lean on the nut to do it up as first time removal has been the worst. The locking tab in pic may be non standard but one should be fitted.

I’ve removed and refitted boxes and drop arms a few times, and I don’t recall any lock washers with lugs as long as those. I’m now going to have to crawl under mine and have a gander now though. I did notice the box was wet the other week though...

The shaft splines and the arm hole are tapered so when nut done up it goes tight, real tight. [ if it was you who removed the arm you will know how tight they can be] The security lock tab is standard fit. There is a master spline and if not...

Should be a large locking washer similar to a wheel bearing lock washer. That nut doesn't look far enough on in the photo. Welcome to the forum.

I would like to see at least one or two threads showing after it’s fully tight. What’s that plate thing that’s on it ? Is it some kind of locking plate for the nut, as I don’t think it’s standard !!

How far on should a steering box drop arm go, there are still splines visible above the arm and the nut is flush to the bottom of the threaded area, I tried to pull it off again and it’s pretty damn on there. but it looks wrong and I don’t trust...