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"It was used for recessed head aviation fasteners.
We used the same sort of fasteners on the Gas Turbine pods that housed the start units for the F-4 Phantoms.
The Phantoms did not have an on-board start unit. They were started by trailer or pod mounted gas turbine engines (basically a small jet engine about 5 or 6 feet long). If the Phantom was flying into a field that did not have a Ground Support Equipment unit that kept the gas turbine start units, they had to put a pod mounted gas turbine under the wing so they could start the Phantom when they were ready to leave.
The pods and much of our gear Ground Support Equipment (GSE in the Marine Corps) gear used the same recessed head aviation fasteners that were used on aircraft. GSE was n=known as MARS in the USAF and the Navy called it Yellow Gear because of the color scheme used on aircraft carriers.
We used the same sort of fastener keys as those made by Camillus. That was over 40 years ago, so for all I remember our fastener keys may have been made by Camillus."