I was just passed down my mother’s five-stone anniversary band. It is stamped 14k TB on the inside.
In between the 14k and TB, there is a sideways diamond shape.
There is a company called Thornton Bros established in 1934 but their mark is a TB inside a diamond shape. Here is a listing of possibilities but none of them seem to match your description.
@Diana McGraw, we can’t tell you how much it would sell for. Your best bet would be to look at sold listings on Ebay of rings with similar gold weight and price it accordingly.