In message . li, Tom
Anderson writes

And does anyone else think that trailer looks like it folds open?

Yes - it was shown in the open position on BBC TV London News a couple
of days ago, and I'm pretty sure that each side was shown folded down.

At a guess, I'd say that the A and B markings are something to do with
keeping the load evenly balanced, as it is only a single-axled trailer.
--
Paul Terry