For what it's worth, OED describes this as an "overcoat of a particular shape formerly worn by men. (Still in slang or humorous use.)"--although it defines it in the singular. Some of the examples use the phrase "upper benjamin."
I got the book "The Pleasures of Peacock"from my public library. It is a collection of all of Thomas Love Peacock's stories. Besides benjamins, his stories are a treasure trove of archaic words that I have been unable to locate from any source. Even if his writing isn't to your liking, he will have looking things up constantly. Check him out and happy hunting.
I searched for "benjamins sir telegraph peel" and got this section from "The Contemporary review, Volume 25" in Google books, but I'm not sure it'll help:
"The heroine of " Melincourt " is Anthclia Melincourt, the heiress of Melincourt Castle and ten thousand a-year, an orphan as coy and with as many suitors as Penelope. Like Peacock's heroines, she must be won, nolens volens; and so, at the time of the story's action, a fashionable dame from town, and an elderly and allied country squire, are located at her modernized " Eagle's Nest," to receive the various suitors who may be "coming to woo." It gives a keen sense of the lapse of time since Melincourt first appeared to read of one of these, a four-in-hand man. Sir Telegraph Paxaret, " proceeding to peel" after a cold day's drive, and " emerging from his four benjamins, like a butterfly from its chrysalis." Not that Sir Telegraph is Peacock's hero, or Anthelia's hero. These are not identical, the second being a certain Silvan Forester, of Redrose Abbey, apparently a philosopher of the Escot type, who burrows for days and weeks in an old cemetery, in the hope of finding giant skulls to bolster up his theory of deterioration, yet withal a kindly philanthropist, who carries out the rales of the Anti-saccharine Society himself, but takes care that his tenants and cottagers shall be housed and fed, and let live like human beings."