I hadn't seen this before, but trying to find more examples on www turned up this, an axe with similar hook (on the Wikipedia battle axe page.
Belt hooks are common in Japanese weapons/tools, being usual on tobiguchi (fireman's axes) and jutte (opinions as to the main purpose of the hook on jutte vary, but they were used as belt hooks).
Can anybody recommend an English-language source which discusses (or mentions) belt hooks on European weapons?


Mace with belt hook