Since the label on the trailer states 50 psi, that indicates the trailer came with load range C tires. Either you or someone else ( perhaps even the manufacturer ) installed a higher load range D tire.
Max inflation pressure for LR C is 50 psi
Max inflation pressure for LR D is 65 psi
If it were mine, I would ensure there are snap in metal valve stems, and run 65 in them.
Someone is about to come on here though, and pick a fight with my statement ;)
Let the debate begin. It's gonna run the gamut from, "your wheels are going to explode", to "you should bought a one ton dually, and put load range G tires on it". Or something like that.