The November, 1969 Gun World article posted above gives information.

Per the article, Cash carried a Mannlicher Schoenauer "Standard Carbine" with 20" barrel chambered for .30-'06 with double set triggers, wearing a Redfield 3-12 X variable scope on Redfield mounts in a stock with a "Monte Carlo hump" that was available through Stoeger at $219.95 "or so", but allowed that the double set trigger was an additional $10.

Images from 1962 Stoeger catalog of MC and MCA models:

MC (Monte Carlo) had a higher 'hump' than did the MCA (Monte Carlo 'all purpose').

Humps: https://www.youtube.com/watch?v=VJg4rwDkkBA