The Highest Possible has been a very elusive item, perhaps because so few were ever made which then had to survive the next 100 years. I have no exact records of the production figures but the Collectors Guide to Air Pistols by Dennis Hiller suggests that the highest serial number known is 1052 suggesting only 1000 odd were ever made.

Engraved on the L.H.S of the air chamber "WESTLEY RICHARDS HIGHEST POSSIBLE - AIR PISTOL". On the frame near the sear is stamped the serial number, 369, which also appears on various other parts. On the left side of the frame is engraved "WESTLEY RICHARDS & CO. - LONDON W" and near the action latch appears "PATENT 24837 1907". The pistol has black chequered vulcanite grips and a 9 3/4 inch rifled barrel. Rear sight appears to be adjustable for elevation only. Length from tip of barrel to base of butt is 12 1/4 inches. The above design was patented by E. Anson, a relative of W. Anson of the well known partnership Anson & Deeley. The "Highest Possible" was also available nickel plated. Highest serial number seen is 1052. Inside of grips are usually scratched with the serial number of part of it and the rifling appears to be anticlockwise. ( From Air Pistols by Dennis Hiller)
The above sequence of photographs shows the cocking of the air pistol.

Paula Rhodes on June 27, 2014 at 6:28 am
I have a passion for pistol. I mean I love to buy false pistol just for fun! This is really a cool stuff.
Simon Clode on June 20, 2014 at 11:19 pm
You can indeed but I am afraid I am no expert in these!
Matt Billington on June 20, 2014 at 11:13 pm
Hi there
I have an air pistol with the same maker markings.
Can I send you some pictures so you could tell me more about it
Thanks Matt Billington
Simon Clode on July 20, 2014 at 6:20 pm
Thank you for writing. I am afraid I am not familiar with Shimwell Bros, I have never noticed the name in our ledgers but I haven’t really looked either! I assume you have matched the air pistol and the story of the company importing the firearms and motor cycles sounds correct. As a company we would I am sure, have stamped our air pistols with the name if that was requested. Sorry not to be of better help!
Simon
Ronald Fernandez on April 25, 2014 at 1:39 pm
Mr. Clode,
I am honored to be in correspondence with you, the owner of the iconic and legendary Westley Richards Company. When I first posted my comments regarding your pristine Highest Possible air pistol, I had no idea that you owned Westley Richards. I had erroneously simply assumed that you were just another vintage airgun collector, given that I had crossed paths with your website as a result of my great enthusiasm for the Highest Possible and other superlative British vintage air pistols, from which I derive pleasure in doing research over the internet. Only after I had read the other superb previous articles on your website, did I eventually learn about who you truly are. No doubt that a Concentric Highest Possible shall one day return home to its creators, whom are also the most rightful curators of such fine sculpted works of art.
Thank you.
-Ronald