[JSeery]

The 221 and 239 cubic inch blocks can be differentiated by the center water holes in the cylinder head decks. From Fred Mills document.

This is not an area I am all that knowledgeable in, but I believe the shapes of the water holes between the two center cylinders indicate it is a Mercury block. So, I would say 1941 Mercury. But you would still need to check the bearings and crank dimensions.

The photos (from Fred Mills) show a block with factory relief.