How can I fix the pivoting buckle clamp plate on my M50 pro mask, which won't stay locked when trying to tighten the head harness?

If the mask can't be fixed at a company level it needs to be sent to the CSMS.

This is not equipment you want to try and fix yourself if you don't have the proper tools. You are dealing with a mask that may potentially save your life.

Get with your supply Sgt and put in a work order so it's fixed properly.

That mask needs to be turned in to the Company CBRN NCO and exchanged, unfortunately there is not a way to replace that locking part, they will need to get it exchanged through supply, hopefully you are not at fault for this breakage, if you are you will have to pay to replace the mask.

The only thing I can think of would be to take a pair of pliers and slightly squeeze across the axis of the pin that allows the clamp to close. It might make it tight enough to remain closed.

But, if you break the clamp, then you could Dx the mask.

Better check with someone before you try