https://www.diabetes.co.uk/can-people-with-diabetes-give-blood.html
Says for reasons not to be able to give blood:
"
- If you have had complicated dental work: over time, prolonged exposure to high blood glucose levels can damage the teeth, giving people with diabetes a heightened risk of needing complicated dental work such as a tooth extraction
"
Which led to my slightly tongue in cheek comment that donating blood apparently makes your teeth fall out.
I would guess that children of the '50s and '60s will often have had extensive dental work because the dentists were paid per filling and encouraged by their training to drill and fill.
These days dentists are much more relaxed about the whole thing because small amounts of decay often don't progress and drilling and filling can do more harm than good.
I am still not really making the connection between extensive dental work and blood donation, unless extensive dental work is taken as a sign of poor control.