If it's not legit, it's most definitely a KO, as Megatron has
never been reissued in US packaging, or reissued at all in the US and Europe. The only legit ones are from Japan, even in vintage-style Japanese packaging with the appropriate logo and his ID number 16 on it. He was offered as a mailaway, twice, but the last one has a special sticker denoting it as such.
The thing that worries me most is that none of his stickers are factory-applied, a tell-tale sign of fakes. I'm looking at images of packaged samples right now and at least some have the chest sticker, and the large

on the grip (By the way, I've also found ones with the tip of the barrel painted orange, or with an orange plug), while others don't. Then again, KO's of G1 Megatron of such nature haven't been reported, probably due to gun laws putting a wrench into the process, but regular KO's are out there.
I would exercise caution either way, not just because of it possibly being fake. You know why.