"High" water pump engines have a pump with a cast iron pulley that's pressed onto the pump shaft, and the pump bolts on almost touching the thermostat housing. "Low" pumps have an extended shaft, and a stamped sheet metal pulley that bolts onto the pump hub. If you're looking at a bare block, the high pump blocks have two holes in the front of the block near the top for coolant to flow from the pump to the block. Later models have a big single hole (about 3" in diameter) and the pump impeller goes back into the block. High pump engines have the impeller inside the pump housing, with a sheet metal cover over the impeller. Pre-68 Dave's adapter plate replaces the back plate on the 216 style pump and allows the high pump to be installed on a low pump block. It requires two holes to be drilled and tapped into the front of the block, but it's an easy fix.
Jerry
