I was so busy being a smartass I forgot to plunk my candidate down on the table. I think there's a decent chance it's Louis Rothman. Here, for once, I don't have to prepare a deep dive since Wikipedia has been kind enough to do it for me:
Louis Rothman - Wikipedia - https://en.wikipedia.org/wiki/Louis_Rothman. And as a famous figure in the tobacco (and especially cigarette) trade there are of course many other sources available online about Rothman's life and career.
What can not be as easily found, however, is an example of Rothman's sponsor's mark. Below is the London hallmark registered by Rothman in 1903, drawn from John Culme's magisterial work
The Directory of Gold & Silversmiths: Jewellers and Allied Traders 1838-1914. Unfortunately I don't have a corresponding entry for Birmingham (the records there have never received the Culme treatment) but find this very suggestive:
Also drawn from Culme's work is a mini Rothman write up (one of thousands such) that he prepared:
A few quick observations. First I find the oval more persuasive than the Rosenthal rectangle; second we know that Rothman was prominent in the tobacco trade and that his Pall Mall shop sold pipes; by contrast I found nothing to connect Rosenthal with tobacco or pipes. None of that is, as the lawyers say, dispositive. But since we're guessing anyway my money's on Rothman.
By the way, I assume there's nothing else stamped on the briar itself? I ask because worthpoint yielded another pipe with an identical maker's mark, assayed in Birmingham. And this one appears to have scratches on the shank that hover just a hair on the far side of legibility: