Anyone have seen similar bark on some regular green maple before?
Won't be sure if it is a JM until the leaves mature in spring but, given the label I'd be pretty confident.
Bark, leaf, size, shape - everything is up for grabs when Japanese maple is grown from seed. I've grown a lot of seedlings and seen the whole gamut of all different combinations of traits and then some.
A grower who knows what they are about will only label seedlings as JM no, no matter where the seed comes from. Those who don't know or who are unscrupulous will often label seedlings with the parent name or at least those seedlings that show similar traits which is just plain wrong.
Whatever you have is a Japanese maple. If it has been grown from seed and has desirable characteristics you are entitled to give it a new cultivar name. If enough others are happy with the new characteristics it could become accepted as a new cultivar.
For now, just grow it for what it is - your very own Japanese maple.