Hey, everybody, guess what I just found! I just looked on page 6/B5 of the 1968 Factory Assembly Instruction Manual and View C of the drawing shows both left-hand side mounts and their screws (GM #274430) as "PART OF ENGINE."
This validates john302, Sonet, and Charley that these left-hand mounts and screws were already on the engine when it was painted red-orange. Woohoo!