Most likely its a refrigerator access door. It can be made up of 2024 alumanum (probably .40") with the perimeter bent over in a small 90 degree bent to fit the recess. I think my '63 has a piano hinge on the left side. Use a standard rv access locker lock (I think VTS (vintagetrailersupply has them).
A simple metal brake is all you need... OR... you can measure the dimentions and call Frank at Frankstrailerworks.com and have him make you one.
Take care!
Marc
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