http://mt.google.com/mt?n=404&v=w2.99&x=%d&y=%d&zoom=%d, which may be intuitively clear to most as indicating that the %d should be replaced by data, but I doubt it. The actual URL I was given (cutting and pasting here" is: http://mt.google.com/mt?n=404&v=w2.99&x=4&y=6&zoom=13.