Thank you for the guidance so far - very interesting that the printing of sleeves would be done by the supplier, and delivered ready to be fitted.
So I need to aim my questions at people who worked at Hellermann (and suppliers of their products) in this era, but as mentioned, if this question was raised 10+ years ago I'd have had more luck with finding people still alive and kicking who were of working age back in the 1950's/60's.
I have just looked into rOtring pens, and that's a whole other can of worms:
https://www.rotring.co.uk/heritage.html - and Indian ink...
https://en.wikipedia.org/wiki/India_ink
Any hint as to the specific type of rOtring pens you recall being used would be appreciated. If I go down this route, I will need to improve my handwriting!
Many thanks again, Scott.