Luftwaffe unknown "Feind hört mit" box

[beutegermane]

A friend has this on his attic, but we both don't know what it is.
Anyone able to shed some light on this?
Any help would be greatly appreciated.
PS, sorry, this is the only picture I have so far.
<a href="https://ibb.co/3fy8wfQ"><img src="https://i.ibb.co/rbGLgbN/lw-kast.jpg" alt="lw-kast" border="0"></a><br /><a target='_blank' href='https://nl.imgbb.com/'>bol com afbeeldingen</a><br />

[Foxy]

Seibt ER1 (ER1a) receiver, empty case. The "Feind hört mit" is not usual for the receiver, suppose a subsequent "refinement".

[beutegermane]

Thanks a lot!