It is common enough in the Mid-Atlantic, including DC; haven't you ever wondered about the telephone ads and instructions, like for automatic redial, calling it the "pound sign"? I haven't seen it as much in the last few decades as I used to, but that is mainly because more signs and labels and so forth are printed, and more memos and stuff typed (emailed), than in the "old days"; there isn't as much need for extreme abbreviations, which is what this amounts to, as there was. For that matter, I haven't seen it being used as an abbreviation for number, as in #5, much recently either.

Never heard it before. I actually paused and came back to look at the comments here before continuing just to make sure I wasn't crazy.