Reinforcement behind waterborne radiator

[hassl]

Removed a waterborne radiator (installed underfloor heating) and now when I'm about to mount a shelf with a molly plug (single drywall) it feels like there's some kind of reinforcement in the wall where the radiator was.

What is usually done there? House built in '92.

Is it just double drywall installed locally behind the radiator?

 

[Demmpa]

In our house from '69/'70, we had a "planka" behind the workbench top. After renovation, we have the same "planka" behind OSB & drywall.

 

[hassl]

Planka as in something wood-like? A bit confusing how to deal with screws in the wall

 

[Demmpa]

Yes, made of wood, the radiators are/were fastened with French screws


Edit:

Picture from bathroom renovation, the plank is 1.5-2 cm thick, 10-12 cm high.