A company I was with had Retenoir's & East. You had better not get caught chaining to anything but those J plates. I too worried about them moving, but if you could keep them in place till you got tension on them I never saw one move even with no stop. Keeping them in place until then, as well as keeping the weight of the chain from flopping the entire thing out of the track on the opposite side of the trailer prior to tighening did indeed take a little finesse. We were trained to use them as the picture shows.
We have two MAC 53' tridem step decks. I HATE pulling those trailers because we don't have those fancy plates you have. I'll have to get my boss to buy us some so we can try. Knew them slots were in the floor for a reason but didn't know what they looked like or what they were called!