I have two DIN specifications with matching Wk numbers, and I am trying to find the source DIN/ISO standard. An extensive search by two of us has so far not found anything. They are
Perhaps what confuses you is that Werkstoffnummern are not used exclusively by DIN standards.
Indeed, there are at least two additional German institutions that employ the same designation, i.e. VDEh (Verein Deutscher Eisenhuettenleute, German for "Iron and Steel Institute") and VdTÜV (Verband der Technischen Überwachungs-Vereine, German for "Association of Technical Monitoring Clubs"). Both have their own material specifications, named respectively SEW (Stahl Eisen Werkstoffblatt) and WB (Werkstoffblatt); and in this case, they both cover the materials you are looking for:
Actually there is also a third one, the BWB (Bundesamt für Wehrtechnik und Beschaffung, German for Federal Office of Defense Technology and Procurement). In this case, the specifications are WL (Werkstoffleistungsblätter) and to my knowledge they cover aerospace material grades.
What you are identifying are the alloy compositions. The Wk# should correspond to the alloys you associated them with, but the alloys could be used in many different specifications based on product, form, and function. Wk# alloys can be referenced in DIN, ISO, and EN standards. You will need to do the legwork to find the application standard that includes your alloy.