Hi Tom,
I guess most of these did end up in several fire places
TT: Your story makes sense, as Kassel was home to a number of technical companies who produced engines, engine parts and also the famed ME262, the first jet aircraft. I wouldn't be surprised if rfo was the code for one of those Kassel-based companies.
Richie: The box was relatively cheap, but the real bonus was the shipping label, which could shed some light on the origin of 'rfo'.