Steve, have you found any information re the Philipsons? Like recent address etc. My searches have turned a blank.
The web of craftsmen involved in making a double was and is extensive in most gunmaking centers in Europe. This tends to deflate the notion that "in house" is somehow better.
Most gunmaking centers by custom allow craftsmen to have a second job. I have seen this in th UK, Spain and in Italy. When I ordered my SXS that was based on ideas set down by Gough Thomas, therefore not doable with the usual machining, I looked around for a willing maker. Castellani took on the job. However, the actioning was done by the Zanotti actioner. Bareling was done by someone else, engraving by Poli whose dayjob was at Beretta, the stocking by a lady who worked in her house, and the finishing by Castellani senior. This was usual in Gardone in the 80s.
The point is who takes responsibility for the finished product and carries it through proof and does any warranty work. That person/firm is the maker and entitled to stamp their name on it. Nothing wrong with that if we all know the score.
The best Holland I ever handled was reputedly built for a royal.It was displayed in an antique "salon" who allowed me to remove the locks and photograph the gun. The locks were stamped ASHES, the Brazier trade mark. I would bet that the barrel blacking was done in Wolverhampton and the color case hardening by Saint Ledger the elder. Still a Holland though!