The single-cherry payout is usually ascribed to the Mills Chrome Bell, of January 1939, at least according to the advertising blurb of the day. This is the machine which the 'jobbers' came to call the 'diamond front', but it IS a genuine Mills product, not an 'after-market' conversion. It WAS the machine which set Pat Buckley on his journey from repairer to maker, though.
The difference in the mech. is simply the length of the payout finger. If you shorten it (or otherwise bend the vertical lever) so that the 'notch' comes into play between the first two payout discs, rather than the second and third, it will pay for single cherry/two cherries, rather than two cherries/two cherries and bell or lemon.
ALL the Mills mechanisms I have seen have been punched for bell and lemon in the cherry position on the third disc, even the ones on 'single cherry' pay; so I conclude that the 'old' payout remained an option.
Can anyone look inside a genuine late Hi-Top, to confirm if, or not, the third disc is still so punched?