I have very little experience with Rugers, but have become aware of a rifle for sale described as the "original prototype for the rifle that became the Ruger .375" this rifle is a left handed 20" barreled action mounted in an original M77 style wooden stock. Rifle includes letter of provenance. Any guesses what it would be worth?

Not worth more than the price of a similar tomato stake.

Google "ruger collectors forum" and ask there. They will know. And if the rifle is legit they will probably know of it.