In the 1922 the Parker Pipe Co Limited was formed by Alfred Dunhill to finish and market what Dunhill called its "failings" or what has come to be called by collectors as seconds. Previuos to that time, Dunhill marketed its own "faillings", often designated by a large "X" over the typical Dunhill stamping or "Damaged Price" with the reduced price actually stamped on the pipe.
In 1936 Hardcastle Pipes Limited sold 49% of its equity to the Dunhill company, and at about the same time a new pipe factory was opened at Forest Road, Walthamstow, E17. In 1946 the Dunhill pipe factory in Notting Hill was moved to Cumberland Road, E13, and in the same year the entire ordinary share capital of Hardcastle Pipes Limited was obtained. It was not, however, until 1967 that Parker Hardcastle Limited was formed, incorporating the Masta Patent Pipe Company.
It is evident through the Dunhill factory stamp log that Parker and Dunhill were closely linked at the factory level through the 1950s, but the difference was not limited to a few minor flaws that made a pipe destined to be a Dunhill suddenly a Parker. Most Dunhill "faillings" would have been graded out of the Dunhill line after bowl turning, and prior to stoving, curing, carving, bit work and finishing. Due to reducing costs in order to for Parkers to meet their intended price point, they would not have been subjected to the same rigorous process and care as the Dunhill production was.
After the war, and especially after the mid 1950s the differences between Parker and Dunhill became even more evident, and with the merger of Parker with Hardcastle Pipe Ltd, in 1967 the Parker pipe must be considered as an independent product. There is no record of Parker ever being marketed by Dunhill either in it's retail catalog or stores.
Parker was a successful pipe in the US market during the 1930s up through the 1950s, at which point it faded from view in the US, while continuing to be popular in the UK. It was re-introduced into the US market in 1991 and is also sold in Europe.
John Loring states in "The Dunhill Briar Pipe - 'the patent years and after'" that while he has seen several pre-war Parkers and some from the 1950s that appear to be Dunhill "faillings" during the finishing process, briar was also obtained from sources other than Dunhill. Loring knows of no way to distinguish the briar source when looking at either Hardcastle, Parker, or Parker-Hardcastle pipes. And that aside from a very few Parkers from the 1950s and before, and a few ODA shapes, we should not expect to find any actual Dunhill production in these lines. While one might be there, it is doubtful we will ever be able to determine it.
Prior to Word War II, the possessive PARKER'S stamp was used. However, at least some pipes were stamped with the non-possessive as early as 1936.
Like Dunhill, Parker pipes are date stamped, but differently than Dunhill. The Parker date code always followed the MADE IN LONDON over ENGLAND stamping. From 1925 to 1941 the date code ran consecutively from 2 to 18 advancing by 1 each year.
There is no indication of a date code for the war years. Parker was not a government approved pipe manufacturer, while Dunhill and Hardcastle were. During the war years Parker manufactured the "Wunup" pipe made of bakelite and clay. A Parker pipe with a 19 date code has been reported, indicating there was perhaps some production of briar pipes as well, but no dating record.
From 1945 through 1949 the Parker date code runs from 20 to 24 and from 1950 through 1957 it runs from an underlined and raised 0 to an underlined and raised 7.
A little help here from anyone with date code information beyond 1957 would be most appreciated.
A letter attributed to John Campbell, Alfred Dunhill Pipes Limited, and posted to A.S.P.  The Dunhill Briar Pipe, the Patent Years and After, by John C. Loring Rare Smoke, Richard Carleton Hacker