I recall seeing a variant of the "slapping number" clock on numerous occasions. The difference being that the clock was ordinarily a standard electric clock but below it an advertising page for a local business would be displayed and at regular intervals that page would be covered/displaced by another advertiser's page.

I have watched people in a coffee shop staring intently at the display and arguing/guessing which page would display next or betting on when a given ad would reappear because in some cases the merchant might have been persuaded to buy more than one leaf and therefore his page would appear two or more times in a cycle.

I never engaged in such foolishness myself because I knew that there were 17 pages and that they were only changed every 6 months and no more han 3 pages were ever sold to any single merchant and....errrrum I only once ever bought a page