Mine was a 0.9mm grub. My Impact is a MK1, Gen. 2 (before the Impact X). Looks like they went from 1.0 to 0.9 as the 1.0 is a really difficult Allen key size to find.
Then they went to no grub screw at all. Check first, if there is one in there DO NOT remove the hammer stop with pliers. It will gouge things up.