Basis in inherited royalty interest

I am preparing to sell a royalty interest in Martin County that I inherited from my parents. The short question is how do I figure out what the interest was worth in 1975 - hence, my basis for figuring capital gains tax.

The details (if you are interested): My father acquired the interest in 1957. The recorded deed recites $10 as the consideration and bears two 5 cent documentary stamps. My mother inherited from my father in 1974. She died in 1975 but we were unaware of the interests until about a year ago. The first well was drilled in 2012-13.

I can’t find information online as to the documentary stamp tax rate that long ago. Also, I presume the interest did not appreciate much in value until there was at least a colorable plan to drill.

Any help will be appreciated.