No.

 

Thanks.

 

Yes

 

You might get better assistance at the sister site:

diy chat

 

FWIW, I didn't do it.

 

It looks like backing.

 

Sometimes it's not worth trying to understand. Do you think it's structural?

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Considering the wood blocking lines up above the baseboard and under the window perhaps someone replaced the wallboard from water damage or something else and left the trim in place.

 

for a wall mounted radiator like a Runtal

 

I wonder if there was some sort of wainscotting on the wall, at some point. They would have been nailers if that were the case.