Thanks Lagopus, your posts on the topic already were a great help. I guess I will keep on the lookout for a possible book in the future.

Your tip with the serial number already helps. My Rosson would have been produced in 1933 then. Two years after the self sliding opener was patented. That really is a cool way to open your shotgun. You can easily do it with one hand, in my opinion better than the standard side-lever.