Process of elimination. Start your thinking at the
oil pan and work your way up to the intake to carb flange. The part in question is ferrous. It looks like it was stamped or formed, not forged or cast. There aren't many parts in the engine that meet those criteria.
Oil pump pickup. Valve spring shims, valve spring retainers. Rocker shaft endcap. Roller lifter link. There just arent many items inside the engine it can be. Most likely valvetrain related. Pull the covers and check everything up there in great detail.