This letter represents a finger, and is called in Hebrew yod or jod (a hand).
per se [I by itself], i.e. without compeer, pre-eminently so.
If then your I [yes] agreement want, I to your I [yes] must answer, `No.' Therefore leave off your spelling plea, And let your I [yes] be I per se. i.s. let your yes be yes decidedly.
Wits Interpreter, p. 116.
Many other letters are similarly used; as, A per se. (See A-Per-Se.) Thus in Restituta Eliza is called “The E per ce of all that ere hath been.” So again, “C,” signifies a crier, from “O yes! O yes!” We have “Villanies discovered by ... the help of a new crier, called O per se [i.e. superior to his predecessors].” 1666.
Shakespeare, in Troilus and Cressida, 1, 2, even uses the phrase “a very man per se” = A 1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894