There are no universal 'proper' spellings of honorific abbreviations, it's determined by which style guide is being used. Last time I checked two widely used guides, they had:
- APA — Mr, Ms, Dr, Prof. — no period if last letter of abbreviation is the last letter of the full word, so Mister, Miss, Doctor, Prof.essor;
- CMS — Mr. Ms. Dr. Prof. — a period after all of them.
But APA and CMS rules aren't set in concrete, so they may have changed. I can't remember if the AP or MLA style guides have any rules relating to this issue… have feeling AP don't have… so up to individual publications.
Outside the US/Canada, similar rules to the APA's are prevalent, but there's also an increasing shift to drop the use of periods after honorific abbreviations entirely. They're useless clutter, no one puts them after qualification abbreviations - MSc, PhD etc.
BR