For me, if you're just a 'runner,' bookmaking is easy enough. Especially back in the old days you had to pick one out of nine horse in a race. (despite some of the odds horses paid if they won, it was still a score for bookies).

Also very solid were Treasury Balance tickets, and slot machines (whether the old one-arm bandits, or the new push button video styles)

All three of these rackets were so simple to operate.
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Runners still needed a solid, financially secure office they could count on.
Treasury balance tickets were a no brainer so long as you have a good distribution network
slot machines were great, so long as you have a good solid mechanic who could repair the machines, place them, etc.