condorlover1 and xrundog,
I wrote an article about the A.D.P. pipe in the Sherlock Holmes story "Silver Blaze" and of its relation to Adolph Posener, that appeared in the Spring 2019 Baker Street Journal and in the April 2020 The Pipe Collector. Here is an excerpt that will help answer your question:
"Adolph Posener and his brother, David, opened a tobacconist’s shop in East London, at 61 & 61½ Mansell Street, in the 1860s, and sold meerschaum, clay and briar wood pipes with the A.D.P. logo. This trademark used the initials for Adolph David Posener. (Adolph’s middle name was also David. But, I surmise, that originally, the D was included to indicate his brother’s name, as, after David left the partnership in 1877, the pipes began carrying an A.P. hallmarking stamp on the silver ferrule bands. However, they continued to carry the A.D.P. oval logo, stamped on the pipe’s wooden shank, and in the A. Posener & Co. advertisements.)
Adolph Posener was born August 26, 1837, in Zerków, Poznan, Poland. He emigrated to England in 1860, and died in 1922, in London."