Here are two pictures of a couple of MIJ snare drums I have with this same extra ply behind the strainer. It's simply there to reinforce the thin shell. You usually find this on the thin-shell drums with re-rings, but not always.-Mark
Ah, thanks Mark. I've actually never owned a wood shell MIJ snare, only ever steel, so I'd not come across it before. Makes perfect sense though, sometimes those shells can be pretty thin and bendy.