Some people can't afford the cost of welch plugs! I've pulled one motor apart to find the hole filled with RTV Silastic, no doubt relying on the sump pan gasket and the sump to complete the seal! Gad, some people
Perhaps this might have been a field fix that should have been fixed at a later time, but the repair kept working, so it was left.
Bob, as far as international variations go I've got 3 Canadian blitz motors, 1 English Generator motor and 1 US 1939 truck motor and ALL have these plug holes on the sump face. Interestingly, the American '39 motor had these holes threaded at some time in it's life and allen key plugs now seal the holes instead of welch plugs.
Below is a pic of the silastic block. After the Silastic was removed, there remains a plie of rusty crud in the bottom of the water jacket. This leads me to think that the threaded plugs are perhaps a good way to go, clean 'em out evey time!