& as a letter in the alphabet?

According to Wikipedia, the ampersand originated as a ligature in Roman scripts of the letters e an t to represent the Latin word et, meaning and. According to the OED, citing Longman's Magazine (1882), the name ampersand is a corruption of "and per se and," that is, the symbol means "and" all by itself. But it's not a letter and would have carried the sound "et" for Latin speakers just as it has the sound "and" for English speakers, and for both it was a shorthand for the conjunction. Apparently, printers of primers for children often printed the ampersand after zed, which gave rise to the conjecture that the symbol was part of the alphabet. I'd guess its attraction is that let printers represent the alphabet in the three lines of an equal number of symbols.