Here's a precursor to the symbol that Prince went by.  From Woroni, 1975.

From Caveat Emptor, 1972.

Aesop's Fables have handed down a symbolic code: "The hieroglyphic writing of ancient Egypt and of Mexico seem to have been, in some detree, a popularized or exoteric outgrowth of a secret language.  ... A symbol-code was sometimes used by Plato and other Greek philosophers, in relation to Pythagorean and Orphic lore; ... throughout the Celtic world the Druids conveyed all their esoteric teachings symbolically; ... the use of parables, as in the sermons of Jesus and of the Buddha, and of other Great Teachers, illustrated the same tendency; ... through works like Aesop's Fables, and the miracle and mystery plays of medieval Europe, many of the old Oriental symbols have been introduced into the modern literatures of the West" (the introduction to The Tibetan Book of the Dead).

From The Fables of Aesop, 1925.

We noticed an unusual set of ESP cards in the extrasensory newsletter L'Insolite, 1980.  Instead of the classic Zener symbols of hollow circle, plus sign, three vertical wavy lines, hollow square, and hollow pentagram, here we find a checkerboard, a circle with eight rays, three overlapping rings, two V's forming an X, and a hollow rectangle.  We attempted to restore these alternative symbols in a print-resolution file for you to download.

Our own alternative telepathy test (free online and app versions) was commissioned by our friends at the Official Prisoner Appreciation Society, dedicated to the cult 1967 TV series The Prisoner.  In the episode entitled "The Schizoid Man," actor Patrick McGoohan uses a special deck of symbol cards to test Extra Sensory Perception, slightly different from the Zener set.  Exercise your own inner eye.

The geometric rogues' gallery.  From The Judge, 1921.

Here's a lost page from that celebrated book of impossible (yet still somehow solvable) math puzzles: Presumptive Conundrums: Rhetorical Math Questions + Answers.

How can one prove the equation "Girls + Smiles = Peace"?  This perplexing calculation appears in Peace College's 1981 yearbook.

The answer is simple, with matchsticks.  Girls are represented by the XX chromosome, rendered with four crosscrossing matches.  Smiles are represented by a smiley-face symbol comprised of four more matches.  Four matches plus four matches equal the eight matches of the peace symbol.

That's what Presumptive Conundrums is all about -- literary, rhetorical math problems that seemingly have no serious answer or provability.  It's the ultimate puzzler for logical- and mathematical-minded folks.

A sun-tablet of the Blackeet Indians of Canada, from The Journal of the Anthropological Institute of Great Britain and Ireland, 1886.

Here's a precursor to selecting emojis.  From Park College's 1931 yearbook.

"Imperial 'U.S.,'" from The Chronicle, Chicago, reprinted in The Literary Digest, 1899.

From Höllen-Zwang by Ludwig von Cyprian, 1509.

"Badge of Allone as Life-Saver."  From Words to Song of Hugh Allone, Sailor, 1886.

From Jugend, 1906.

Here's the esoteric symbol of the matrix, from The German Pietists of Provincial Pennsylvania by Julius Friedrich Sachse, 1895.