Thanks, Chombi!

I got to some of the cleaning on the GR today, including the fixed cup of the BB. And...it has the same machining marks.

As I understand it, in this era the BB is almost certainly Swiss thread, so while I can see trying to track down a better matching (French) adjustable cup, even one of the inexpensive new ones, no way I am going to try to find the Swiss thread fixed cup. I'm building the bike back up with the original BB. Maybe after I have ridden it for close to another 40 years it'll be worth looking at replacement.