Water jacket? Straitjacket? Something some demented mud-daubers did? A guess at this point, but the circle / plug in dead center is a sand casting hole leading into a water jacket.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
A plug. Behind it is a raised edge that stops the plug from going in further. You can crack the block if you try to drive the plug too far. They have been known, after many decades, to rust through and leak coolant.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
Yep, anytime the bell housing is removed, that plug needs to be changed. If it springs a leak due to rust-through, it's a huge job to remove everything covering it up with the engine in the vehicle. It's a simple cup-shaped disc that's installed by coating the edge of the plug with non-hardening gasket sealer like Permatex and giving the plug a firm tap with a hammer once it's in the counterbore. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
