Mal and Lagopus have it nailed. The usual name is a stalking safety and the configuration on the Purdey is the most common one. Rigby's tended to inlet a small square button which one depressed to hold the safety in place. I have even see one on a Mauser BA with a mod-70 type safety. It was a lever let into the top of the safety lever that froze the safety in on position and was a wonderful little piece of machining. I have only seen this once and it was worth buying the rifle for that alone!