The two piece/split jaw/clam shell are basically the same types. Several brands use the split jaw but place the jaw pivot pins forward or aft of the hitch pin.
My first hitch was a '84 across the bed Atwood (top of both wheel wells) that had the J hook with a sliding bar behind the J hook. I was rated at 12k pull and 2000 load.
I've seen lots of style jaws. Most like wittmeba says were a modified version from another type. Some aren't marketed any more or even the brand is gone now.
IMO what jaw type works best depends on different criteria such as weight of trailer/amount of use/commercial use vs RV use and the usual mine is the best.