I wonder how long to took British Double Rifle manufacturers to get around to putting rebounding locks on these rifles? The .577" Double I used to own had the same non-rebounding locks as yours Mike, and over a Century or more, dozens of people must have tried to open the action without first bringing the hammers back to half cock.
If this wasn't done then one of two things happened, one or both of the firing pins got bent or broken, or, the Firing pin tips caught on the extractor, usually in the corner of the groove for the cartridge rim. The are not so easy to repair once the burrs build up and make chambering a round difficult. The drill is easy to remember, Fired or empty, always half cock the locks first, before opening the action.
A very, very, nice rifle Mike, my congratulations.