A number of books on gambling have illustrated how a slot machine works with listings of the distribution of symbols on the strips of a typical slot machine.
John Scarne's Complete Guide to Gambling gave a specific example, the Mills "Twenty-One Bell", which has pairs of overprinted symbols in some positions. At least one other book on gambling quoted that example. The book "The Casino Gambler's Guide" by Alan N. Wilson gave an example which was unusual enough that if it actually does refer to a real machine, it is sufficiently atypical in its payout structure to be of limited interest for my purpose.
But two arrangements of symbols on slot machine reels have been quoted in a number of books on the subject of gambling.
I have found that the source for the first one is the booklet "The Facts of Slots", and its source is the slot machine provided by the police to a Wisconsin statistics professor:
Lemons 3 - 4
Cherries 7 7 -
Oranges 3 6 7
Plums 5 1 5
Bells 1 3 3
Bars 1 3 3
(unfortunately, I tried to use the Code modifier with these, but it does not work properly, as it removes the carriage returns between lines; I was trying to get a monospaced font so as to align the columns)
He is pictured with a Mills Extraordinary in one photo, so that could be the type of this machine. The arrangement of symbols is close to that of the Mills SP strips, but it is not exactly the same.
This is clearly from an old-style machine, which pays on two cherries, and on two cherries with a bell or a lemon. What I'd like to know is the name of the machine it was used on, or the designation for the set of strips.
The same is true of this arrangement, about which I have less information:
Cherries 5 7 3
Oranges 4 1 10
Plums 6 1 4
Bells 1 9 1
Horseshoes 2 1 1
Bars 2 1 1
It's clearly for a new-style machine, which pays on one, two, or three cherries. Some sources give "Horseshoes" for the symbol so shown, others give "Melons". Also, at least one of the books in which I saw this arrangement came from the UK.
It's very similar to the V-12-70 set of reel strips from Jennings.
In the book "Slot Machines" by Dieter Ladwig, a postwar machine by Jennings, the "Standard Chief Bell" is pictured that does have the horseshoe symbol in addition to the usual slot machine symbols excluding the lemon. However, the payout schedule for that machine, if applied to this arrangement of symbols, would result in the machine paying out over 9000 coins for every 8000 put in, thus losing money. He notes that the machine was also made for export.
Thus, two of the most often cited sets of reel strips, back in the days when mechanical slot machines were in active use instead of in the hands of collectors, are mysterious, but I'm hoping someone here might be able to settle the details so I can share them on my own web site.