What are the two holes with slotted plugs in a Honda 300 EX for?

These two holes are called inspection holes. The larger one is there so that it is easy to turn the engine by hand in order to adjust the camshaft valves. There is a large nut inside which you can put a socket head on in order to turn or crank the engine crankshaft over. The smaller hole will give access to the T (time) mark on the engine's flywheel.