WW1 Imperial German belt hook, for the WWI M1910 tunic they’re specifically called the rear “button” hooks the visible face is cast to look like a crowned button rather than an obvious hook.

German uniforms used four hooks in total to help support the weight of the belt and its equipment. Two were straight side hooks, and two were the rear “button” hooks, all sewn into the tunic itself to better distribute the weight.
The hooks were sewn to a piece of heavy wool shaped like an upside-down “U,” cut about half an inch larger all around than the back plate of the hook. The hook plate was then sewn onto that wool patch, and the patch sewn onto the inside of the uniform, with the hook protruding out through the seam.
The rear “button” hooks are the rarer and more interesting of the four, because unlike the plain side hooks, they were deliberately made to look decorative from the outside. This has the Imperial Crown motif, which is correct for the Prussian state tunic – the standard crown button design used across most German states during the Imperial period.
The M1910 Feldrock was the standard service tunic of the Imperial German Army at the outbreak of WWI
The heavy weight suggests it’s the  iron construction version and it retains a lot of the original paint possibly excavated.