Hold-down safety Sheridan Air Rifles.
These rifles were made between 1949-63. With thanks to Bryon for these pics.
Tom Gaylord, in the Pyramid Air Report, said:
"The first safety device Sheridan used was an automatic safety that airgunners today refer to as the 'thumb safety.' It was an attempt at making certain the shooter's hand was correctly positioned on the grip. Otherwise, the spring-loaded thumb button could not be depressed, and the gun could not be fired. There was just one problem. Sheridan positioned this safety about one inch too far forward on the wrist of the rifle for the vast majority of shooters. They could not reach the thumb button without holding the rifle unnaturally...
"As a result, a great many of these thumb safety guns have been altered with field fixes to permanently hold down the safety. The easiest method is to jam a wooden toothpick in the slot on one or both sides of the safety button. A very common fix is to drill a small hole in the thumb button for a crosspin so the safety cannot rise out of the stock. The safety was hated so much that relatively few unaltered guns survive today. (Altered guns lose much of their collector value.)"