The main difference between Proprietor and Proprietress is that the Proprietor is a one who owns something and Proprietress is a Wiktionary redirect
A sole owner of an unincorporated business, also called a sole proprietor.
One of the owners of an unincorporated business, a partner.
One or more persons to whom a colonial territory is assigned, like a fief, including its administration.
“From 10 September 1621 till 12 June 1632, Sir William Alexander, styled Earl of Stirling and Viscount of Canada, was proprietor of the Scottish colony Nova Scotia.”
A female proprietor.
the owner of a business, or a holder of property
“Chinese restaurant proprietors”
“by 1640 he was himself the proprietor of a three-hundred-acre tract”