Its either a m2.5 or m3. If your Attu has the AFC right next to the air hole, see if the grub screw fits the full hole. If it does, it should be a m3. If the grub screw is too big, your full screw is m2.5. You can get stainless grub screws from
http://www.fmwfasteners.com and I also found stainless Philips head on eBay and Amazon. I would recommend 6mm length.
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