Bob, my '79's original speedometer has this (green) line. My bike was built for the German market and was later imported when the original owner emigrated to Canada. The replacement speedo which was probably bought in Canada has no line on it.
Most city speed limits up here are 50 kph (approximately 30 mph). I'm guessing it's the same in Europe. The purpose of the line was probably to highlight where the speed limit is.