Designed and used by Public Safety Divers. Whether you are on surface supply, or S.C.U.B.A. you should never get into the water without a redundant air source. This switch block allows the diver to choose between a primary air source and one that is completely redundant. This GSB is the most widely used by FFM divers than that of any others. Made in the USA and built last. Now available as ambidextrous.