Well, it's a very nice bike. My guess from the serial number on the seat lug is that it was probably built in 1973.
I can't find any reference to a "Champion" West German brand on the usual web resources like
Classic Rendezvous, so it may have been a small builder.
One suggestion I have is to contact
Wastyn's in Chicago. They've been around for a very long time and should know what you've got.
Good luck.