EDITED (The history behind this has also been discussed previously on these forums incl https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=54629&page=1 )

I happened to be in London the other day and called in at Purdey. During a discussion the gentleman in charge of the gun room we turned to stock finishes and techniques. He pulled out a copy of Lt Col. Peter Hawker's book and flicked to Manton's recipe Salopian once posted in these forums. He went on to recount the story behind the Purdey finish. Apparently a very long time ago they employed a gentleman by the name of (Harry) Lawrence. Over time they noticed that the finish on the stocks he did lasted a lot longer than others. It turned out he was using his own formula. Purdey tried to buy the recipe. However, Lawrence refused. He did, however, agree to sell the bottled finish to Purdey. This continued until his son, on his retirement, agreed finally to sell the formula to Purdey. To this day it remains a closely guarded secret with just a few trusted people knowing the formula which is locked up in a safe. Apparently they make it in batches and various batches develop a reputation within the company for greater or lesser quality. Those in the know at Purdey, when shown the recipe in Hawker's book, responded "no that's not it." Unfortunately, I couldn't recall the recipe Salopian said had been attributed to Purdey to further the conversation although, in any event, the gent I was talking to was not one of the few 'in the know' anyway.