Jno is the common abbreviation for John.

I've seen references to it being short for Jonathon or the Latin variation Johannes.

I've never seen a reasonable explanation of why the 'n' comes before the 'o' - it doesn't make sense for any of the names.

I recall it on shop names - I seem to remember a couple of instances in Strand Street.

Even Jeeves and PG Woodhouse use it.