I only know of one Krefeld in Germany & it is in North-Rhine Westfalia, northwest of Dusseldorf. The address my mother sent in Sobernheim is in the Rheinland-Pfalz region. The person sending the card from Sobernheim ("Fr. Bayer - probably Ulla), would have been about 50 years old in the 1980's. It is possible that she had moved away from Krefeld by then (school? work?).
I am sorry that I don't have more information - it has made my search very difficult, & I understand if you do not have enough information to work with.
Beth