Thanks everyone for the input. Virtual Ts all around.
The top one is a standard K+M clamp with the international standard 3/8" Whitworth thread.
The bottom one looks like it also has the standard 3/8" Whitworth thread - but with a 3/8" to American 5/8" adaptor screwed to it.
If you put a 3/8" > 5/8" adaptor on top of the Whitworth thread will it completely cover the bottom, non-threaded, portion of the Whitworth thread as well? I like that one but wouldn't want that little bit of non-threaded part sticking below the adaptor.