Main Difference
The main difference between Proprietor and Proprietress is that the Proprietor is a one who owns something and Proprietress is a Wiktionary redirect
Proprietor (noun)
An owner.
Proprietor (noun)
A sole owner of an unincorporated business, also called a sole proprietor.
Proprietor (noun)
One of the owners of an unincorporated business, a partner.
Proprietor (noun)
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.”
Proprietress (noun)
A female proprietor.
Proprietor (noun)
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”