Euroliner or not

milodon:
Yes, they are only locked at the bottom. Still locked though.

So there’s no difference between commonly used UK or Continental trailers. They all only lock into place at the bottom and are held in place by friction and the side boards.
If the side boards aren’t in place in the upper slots, the uprights can and probably will move. Which is what Wheel Nut wrote in the first place.