What is this?

[colorfixer]

Here's my $0.02:
-It could be possible that the mask deformed during the heating/evacuation cycle.
-If the tab(s) had detatched from the ring that surrounds the mask, there is likely not that much movement that would occur, and I concur that there would be something rattling around inside the envelope. Tapping the envelope would reveal microphonics. The mask looks really tough in any event.

-What about a damaged or misconfigured gun, with trajectories off or damage to the lens element structure?