I haven't been able to confirm which mask study he shared, though, in part because of the comments in the above Tweet, I suspect it was the recent one on masked children being exposed to ~6x more CO2 than unmasked children. That study is discussed in the threads here, here, and, here.
Did some digging, this seems to be from the Daily Express (July 3rd). See here, for instance.
It is also discussed in this DiscloseTV Tweet.
I haven't been able to confirm which mask study he shared, though, in part because of the comments in the above Tweet, I suspect it was the recent one on masked children being exposed to ~6x more CO2 than unmasked children. That study is discussed in the threads here, here, and, here.