Indeed, there's a lot of history in that suit. It's likely that the "PM" marked on the label was Patrick Mead, who is quoted (as "director of Henry Poole & Company") in
this 1987 NYT magazine article on Savile Row tailoring. Mead left the firm in 1994. The same article suggests that the price of a similar suit in 1987 was "$550 and upward" (my 1984 blazer from John Kent at Hawes & Curtis was about $350) so you're acquiring the suit at 1987 prices with the breaking-in thrown in for free.
While on the subject of Poole, I ran across this British Library
oral history transcript of an interview with Angus Cundey which is a fascinatingly detailed account of his life, his entry into the family business, and full of surprising notes like an early stint at Lanvin, and then Paul Portes, in Paris. There's a lot about the various post-war firms and what tailoring has been like in the intervening years. The interviews were done in 2003.
Felicitations on your discovery – and purchase, if you bought it. If so, let us see it being worn after any needed alterations!
