Proof would tell you most of this. Makers could make what ever they wanted to make, provided there were no patents or if they paid for their use. Often they could bring in "in the white" parts and fit and finish them up as their own. I have seen Purdey thumb openers not made by Pudey and not made to their level of fit and finish. I have a Lindner with this opening system that is made to a very high finish level. I doubt either action was provided by Purdey.