That big core plug centered on the block is a flat design which does not have a shoulder. It is slightly convex-shaped, and the outer edge fits into a shallow counterbore in the block. Once the plug is positioned in the block, a firm tap with a big hammer expands the outside diameter and seals it. Putting a little brush-on non hardening gasket sealer on the edge of the plug helps seal the coolant. The smaller plug that's visible lower down on the back of the block is there to plug the camshaft bore, and seals oil , not coolant.
Jerry
