Well, back to actually considering the question that WD proposed...
The major German steel supplier today is ThyssenKrupp. Back in the 1960's it was probably a bit smaller and several smaller companies conglomerated to form the large one. They are a major supplier of hot and cold rolled steel in Germany, so my guess is that they were at least one of the steel suppliers, and I bet there was a second source as well, maybe a third source such as Saarstahl, Boeker & Wender, and others. There has been a LOT of changes to the steel industry in the past 50 years and all the companies names are changing.
While I have no facts to back it up, just intuition, I suspect that steel recycling (i.e. old military hardware) was not as prevalent in western countries in the 1960's as it is today, and many companies probably specified "virgin" steel. The old joke around here is that old Chevy's get sent to Japan for recycling into Toyotas, as a lot of Japanese steel was recycled and China basically didn't have a manufacturing presence.
