Okay, I see it there. Never seen such a thing on my hammer doubles, albeit most were not back action guns.
I think the grip safety, or guard safety/safety guard, was more common on muzzle-loaders. On pinfires I've only seen them on very early examples, or as a carry-over from a muzzle-loader conversion. Here is another example, from a conversion, but sadly not a back-lock gun...
How these work is illustrated in The Shot-Gun and Sporting Rifle by Stonehenge, 1858. The author points out that leaving a space between the safety and the gun allows for a twig to get in between. The final sentence in its entirety reads "To remedy this inconvenience it is only necessary to make the part which appears outside the stock of solid metal, and let it into a socket cut into the wood." Such a design is the one pictured here.