The T configuration was used as the 12V charger connection on older fire trucks for years. A receptacle was mounted on the tailboard or battery compartment. Most newer trucks have an onboard charger/maintainer supplied by a 120V shore cord. Our club buys and restores old fire trucks, and we have a box of these plugs & receptacles. The older trucks, if you started the truck and drove out with the charger connected it would just pull out. Newer trucks have auto-eject inlets that have a n ejection pin that pushes the plug out when you turn the ignition on.
