All those links seem to indicate that the hole was for bolt disassembly. For years, some folks have insisted that it was, or was also, used for securing rifles in cases or in barracks rifle racks. Maybe, but its primary purpose seems to have been for bolt disassembly, as was the hole in the cup type buttplate. If the stock hole was used for a securing rod, the use of a hole in the buttplate would negate that purpose.
Jim