I looked for "Palmerston suit" in a few databases to which I subscribe and while I found references to Palmerston suits, none explained what that was. It seems everyone KNEW what a Palmerston suit was and no explanation was needed. Some of the things I found, though, were:
-- the first reference I found to a Palmerston suit was in a few 1862 ads, although I found an ad for Palmerston bouquets in an 1860 ad.
-- One 1862 ad for Palmerston overcoats, from a "national tailor" who advertised for sale Prince of Wales overcoats, Garibaldi overcoats, Cambridge overcoats, etc. Most often when I saw ads for Palmerston suits they also advertised suits named for other well-known figures.
-- An 1863 ad for the Ready-Made Clothing department in a business advertised their Palmerston suits with lounge suits, derby suits, etc.
-- An 1865 ad mentioned small boys handing out flyers for places selling wearing apparel, such as with ads stating "...(we)
promise to supply bearer with a Palmerston suit, cut in the latest fashion, and finished in the first style, for the sum of 2 pounds, 2 shillings sterling, payable on delivery"
-- An 1866 ad for caseloads of wearing apparel (one dozen per case?) included cases of Palmerston suits
-- A clip of an 1871 ad is attached here for educational purposes; please do not reproduce it anywhere else, because of copyright
-- An 1882 book (Nut Bush Farm) mentioned Palmerston suits a couple of times: "
As he came closer I saw he was dressed in a loose Palmerston suit, that he wore a low-crowned hat, and that he carried a light cane" and "
As erect as a grenadier...he looked every inch a gentleman -- just in his everyday clothes, a Palmerston suit and a round hat, he was, as many a one said, fit to go to court.'
-- The latest mention I found was in a 1923 ad for a variety of suitcases, including the Palmerston suitcase, covered in 'brown velma fiber.'
I regret that I was unable to find specifics about the suit style.