Laser mirrors 2 tubes

[elmale]

Hello, a laser cutter that we have at work has 2 tubes, but one day cleaning lenses I reached the exit of both and there I saw that a mirror of the 2nd tube was badly burned.
I have not found any information on the internet.
Does anyone have information on how these mirrors are and where they can be purchased?
It is not the 3 normal mirrors that I am referring to but the ones that join the beam.

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