I think the issue here is one cannot identify a maker/retailer only at best a design/patent. It's clearly a snap under lever that uses a relatively full sidelock plate so is limited to patents using that. For example Brebner sold a number of actions patented by others (I have one of his original Purdey thumbhole levers for example). Similarly I have a Lang's patent underlever sold by Patrick that looks very similar to the subject here.
Jeremy