This is a Southco tool/key used with their E3 vise action compression latches. The latch is widely used in RV, motorhome, and travel trailer applications, and can also be found on many boat and yacht compartments.

• Part Number: E3-3-1 
• Head Style: Triangle
• Head Size: 7mm (0.28") 
•  Key measures 2-1/8" inches wide x 2-3/16" tall x 1/4" thick.
• Triangular hole measures 5/16" in. (7mm) wide x 3/8" deep, measured with micrometer.