Hello ,
Gordon Bruce's history of the development of Webley air pistols states that any pistols marked PATENT APPLIED were clearly produced before the patent was granted in August 1924. Others destined for the American market, were stamped USA PATENTS PENDING which implied manufacture prior to July 1925. Similarly, the majority of pistols marked FOREIGN PATENTS PENDING were manufactured prior to December 1925, final date for the Swiss patent. The book some also states that around this time on some examples, the patent markings were not stamped but etched electronically onto the rear right hand side of the air cylinder with Ser No 17312 being quoted as such an example. Apparently, the Patents were also sometimes listed in a different sequence.
So, although quite rare, it seems etched pistols were sometimes produced during this relatively early period of manufacture.
Regards
Brian