When my trailer was delivered, the guy said I needed some sort of "pin" for my ball receiver lock-down thing (I don't know what to call it). He gave me a cotter pin looking thing and said it would work in a pinch, but that the pin I should get is more D-shaped that connected all the way around and back to itself so it wouldn't fall out. I didn't really understand since we didn't have any pictures or idea of what it was. Looking for it at Walmart and Online - everything I see has this really THICK bar, like 1/2" or something, which is way too big for what I think I'm supposed to have. The cotter pin thing looks no bigger than like 1/8" or so thick. Can anyone point me to what I need?