Something similar was done on British civilian masks of the WWII period. An additional 'Contex' filter was added to update the mask against newer chemical agents. Thankfully, those masks were never used for their intended purpose.
There should be an ink stamp on top of the filter, next to the threads, with a date. If not, check the lenses. They should have dates stamped around the rims. That might not tell you when the mask was issued, but it'll tell you when it was last used.
Regards, B.B.
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