That is a Road Racing Drivers Club (RRDC)
decal on the front of this Bob Grossman helmet. The RRDC is an organization whose members are elected by their peers, and to quote the RRDC's
website: 'Their (RRDC's) mission as stated in the original by-laws was: “to improve the lot of the drivers in road racing and the conditions under which they compete”. They worked with track owners by recommending and supporting spectator barriers, runoff areas, modern pits, equipment standards and race control training. Another issue addressed was the unseamly idea of "amateur" drivers receiving compensation for their services. The club also worked to train new drivers as they entered and progressed through the sport.'
Bob Grossman served as president of the organization at one point, as did Mark Donohue.
Check out their gallery
for some great photos of cars and drivers that members of this forum are sure to appreciate.