Now... David Parkers excellent Köti build in 1:16 showed these stickers on the air filters:

NO idea what the text says (something about German regulations I guess) and no better (real) pictures to be found, let alone decals in 1:6...
I would still like to know what's on the stickers but in the meantime I designed my own.
4 prints for 2 engines

Also on 1:6 you are not able to read what it says! So you are the only guys who know now haha!

Attached using double-sided tape

Some varnish to seal it

End-result. Papers turn white on the edges.