Green’s Dictionary of Slang

(UK Und.) a letter.

1821		W.T. Moncrieff Tom and Jerry III v: turn.: Here’s a stiffener for you, Doctor! log.: A letter for me!
1835		G. Kent Modern Flash Dict. 32: Stiffner – a letter.
1848		Flash Dict. in Sinks of London Laid Open [as cit. 1835].