I think of this type of tow ball as a "European" style. (That could be really ignorant, its just where I've seen them).
I've read these described as a "hook". Is that just descriptive of the shape or are they really meant to function as a hook in some situations? Like a "pintle" I think they are called in the US.
For comparison, here's a US "equivalent" which this reminded me of; though this US version is specifically meant to be used either as a ball or a pintle "hook" AFAIK: