Some of the boards were gaffed. A "protection slip" was included to show the operator which holes were winners. Those were punched before the board was put on display.

Two interesting references on punchboards are: the movie "The Flim Flam Man", and the book "Scarne's Complete Guide to Gambling", by John Scarne. This is, IMHO, simply the best introductory book on gambling ever written. Mr. Scarne was not formally trained in mathematics, and figured out the odds calculations on his own. Before computers too. You see this book on occasionally. Well worth the $20 or so it usually brings.