Luftwaffe factory inspection stamps?

Hi all,

Does anybody have a list of Luftwaffe factory inspection stamps? I’ve searched the web but can’t seem to find anything.

I’m regularly coming across a K??? (Within an oblong) on airframe peices recovered from both Me and Ju aircraft. I’ve also recently come across a similar stamp but starting with an R.

Would I be right in thinking it’s maybe an aluminium factory inspection stamp rather than assembly plant?

Many thanks,

Darren