That's the 1/8" NPT hole with the square-head plug at 11:00, just above the timing cover, on all Flint V-8's; the machining process was different for the front oil gallery intersection between Flint and Tonawanda, so Tonawanda blocks didn't have the hole or the plug. That's a key issue on Corvettes, as all Corvette small-blocks came only from Flint V-8 (as did all Z/28 302's). Photo below.