Been a long time since I watched Mysterians. Popped it in tonight and noticed several shots around the 20 minute mark were reused in Mothra vs Godzilla. I've never heard anyone mention this before, but I always had a weird vibe that some military shots in MvsG looked too old to actually be from that film.
Thing is, it's hard to sort a lot of this kind of stuff out, because some of the footage was reused in the '70s too.