When Ben first posted these and Jon Peterson clarified that the small blue line is trailing sales of Supplement I, a few of us pointed out that it's a bit funny to include Supplement I, since it doesn't have any actual setting info, despite the title.
You're incorrect about the sales, though. Supplement I went through 12 printings, the last of them in November of 1979. That was the last print run for all five supplements to OD&D, though II and III only had 9 printings, IV 7 printings, and Swords & Spells had 6.