I have a little battery bok in same size as a cigarette package,on the ID plate you can read.

Batteriekasten.
Baumuster: BAK
Gerat : 126-157A
Werk 1258
Anforderz FL 68418
Hersteller kdf

The box are made for 2x 1,5 volt cylindrical batteries and are probabley for some emergency system in the aircraft,someone seen this type before ?

Regards Norway

Sounds like a battery box for the Notwendezeiger , a small instrument that was used if electric failure.

Best C