“Jno.” is an abbreviation for “Johanno”, the Latin equivalent of “John”. In the days of yore, one’s “Christian name” or forename would have been recorded in the parish register by the priest on the day of one’s baptism, but always in the Latin language rather than the local vernacular.
That Latin name became your legal name, and would appear in deeds, censuses, charters, and other official documents. It would also be used for addressing letters, naming the proprietor of a business on a sign above the entrance, or signing the products of an artisan’s handiwork — like guns.
Over time the most common Latin names acquired standard abbreviations. In many cases these shorthand names were close to their modern English equivalents, so we have no trouble with “Thos.” or “Chas.” or “Ed.”, and in fact still use them. But how many people now know that “Gul.” = “Gulielmi” = “William” ? Genealogists and historians do, but precious few others.
