When the first Douglas A4D-1 and A4D-2 Skyhawks entered service, they only had three weapons stations (one under each wing and centerline) and depending upon the number of external tanks carried, only one or two bombs could be carried, which significantly limited the Skyhawk's conventional weapons delivery role. At China Lake, test squadron VX-5 came up with a solution that used the Aero-15 outer wing weapons pylons from an AD Skyraider welded to an adapter that could be hung from a Skyhawk weapons station- this was called the MCBR- Multiple Carriage Bomb Rack. In 1959 sixteen bombs were dropped in a demonstration by an A4D Skyhawk for the Navy brass at MCAS Yuma.
The Navy immediately approved of the project and Douglas submitted an unsolicited proposal for a more refined device called the MBR- Multiple Bomb Rack. The first units went out to Skyhawk fleet units in 1960.
The next refinement came shortly after- the Navy's MCBR and Douglas' MBR only dropped the bombs and local airflow around the aircraft potentially could cause the bombs to bump into each other, affecting delivery or worse yet, triggering a detonation. A small pyrotechnic charge was added to each bomb rack to provide positive separation of the bomb from the rack, thus creating the MER- Multiple Ejector Rack, which could carry six bombs in two groups of three in tandem. The MER quickly entered fleet use and would be supplemented by the TER - Triple Ejector Rack, a shortened version that only held three bombs.
Source: Strike from the Sea: U.S. Navy Attack Aircraft from Skyraider to Super Hornet, 1948-Present by Tommy H. Thomason. Specialty Press, 2009, p101.