Hi Soren,
All this is pretty damn well done!! Thanks for your help! This confirms that I should definitely have 1'' cylinders (which I do) and that the parts supplier made a mistake. I sent them an email before seeing your post last night and got a fast answer: their fault!
What is confusing about all this is the fact that in many parts suppliers' price lists, you get to think that something changed during 1965, as they mention an "early 65" and a "late 65" brake setup. Is there a trick here because of the "year-model" (this is translated from french, dunno if it makes sense), that can start anywhere during the previous year (usually in summer)? Still wondering...
What you suggest regarding the cylinder diameter is interesting. Can you further develop why you need MORE breaking power (pressure) if the brakes already tend to lock? I would expect it to be the other way round!!
Too bad I don't have 7/8'' units in stock, would have been fun to try a swap!

Thanks for your input anyway! Cheerio! Séb