I spoke to the PR man maybe 10 years ago and he told me that after the war WMF did not keep records of who designed what for them. He also told me that they increasingly sold bought in glass from various sources - though he was not sure when this started. From 1985 they stopped producing glass altogether and settled on a fixed arrangement with Zwiesel to supply tableware. So a WMF sticker is not telling you it was made there - merely that it was distributed by them. Some of their items from the 70s might be Oberglas, others Zwiesel - and some produced by themselves.
WMF has always had a core business of flatware, kitchen equipment and professional coffee makers. And as WMF maintain a strong retail presence in all German cities, "table" is the key word - whereby they (sensibly) refrain from making glass and porcelain themselves.