Most varnish and polymerizing oil coatings will yellow and darken over time. Completely normal and expected. The Behlen's that Dr.Gaddy used and recommended is a nitro cellulose lacquer and generally does not darken with age. It is also easily removeable with lacquer thinner or acetone for recoating. Varnish is not so easy to remove. I'd call Turnbull and ask what they used in those days.
FWIW, I have an LC Smith that Turnbull recolored very early on in his career, and the colors do not seem to have changed in the 15 or so years that I have owned that gun. But, it does not look like it was coated with anything.
