OMG, I remember "Sincerely yours" and "Yours truly" from writing letters as a kid and it would never have occurred to me that "yours truly" is the less personal "more businesslike" of them!

Mind you , writing letters as a kid I was a huge fan of all the yours witticisms that probably all 10 year olds thougt were hil-arious

yours till Niagara Falls
yours till the kitchen sinks
yours till the door jams
yours till the fruit flies