"Bolted Safety - A standard feature on a Holland & Holland Royal / deluxe double rifle.

A secondary catch on the standard safety, often seen on big-bore double rifles, designed to prevent accidental disengagement by a careless gun bearer.

Actually, it is found on many many English double rifles; I have it on one of my Lancaster rifles, and on about four other English makes as well. It was used not only on big bore doubles, but also on medium bore, and smaller bores too. Historically, there was a period of time when the English makers included bolted safeties on their doubles; I think to meet demand for them from customers. Just because they are there, ready to be used, doesn't mean that you have to use them, as you can simply use the regular safety normally, or the bolted one too, when you think it a good choice in certain situations.