I don’t think I’d take one of those (old $100 bill). But it’s been so long since I’ve seen one that I’m not sure I’d recognize one if I saw it.
A new redesigned $100 bill (with added security features not found on other bills) was supposed to be released last year. However, I believe that last year the Treasury lost an ADA court case about currency not being distinguishable by the blind. I’m just guessing, but this may be what’s causing the delay. They may be attempting to incorporate Braille or some other tactile clue on the bill.
I think you’re largely correct. My memories of the period are that there were very few vending machines of any sort at the time (in comparison to today), mostly selling small items like bars of chocolate – the only thing I can think of that might cost as much as 50p is cigarettes. Slot machines in arcades were the old, mechanical type like one-armed bandits and pinball machines, and mostly took only coppers, pre-decimalisation. Some were converted, but I think most were simply trashed and replaced with newer machines through the 70s (a lot of them were practically vintage by that point anyway).
I suspect that there was a period of technological development in slot and vending machines in that decade, which coincided with a sustained period of inflation, meaning that manufacturers had an incentive to make their machines easily upgradable.