From the appearance and your description, it looks a bit like 'Shindeshojo', however I would be very hesitant to assign it a specific cultivar name. With the huge number of registered cultivars on the market and the very close similarities many of them share it is virtually impossible to ID a Japanese maple with any degree of certainty. As one very reputable JM source states: "There are things that are absolute in life such as death and taxes.
Unfortunately being able to visually identify a specific Japanese maple
cultivar with 100% certainty isn’t one of them. There are several
reasons for this. First, your tree may be seed-grown guaranteeing that
it is not a named cultivar. Many older trees that may have been grafted
lose there graft “markings so it is impossible to tell whether they are
seed grown or not. Second, many Japanese maples are similar to each
other in color, leaf type and many other visible ways. The actual
differences in many cultivars can be very subtle. Even with the ones
that are VERY distinctive you cannot guarantee an ID although you could
give a “likely” ID. There are also many differences brought about by
environmental conditions that could also make the actual subtle
differences in appearance even more obscure. Finally, the number of
different cultivars that have been produced over the years is
astonishing and may possibly number a thousand. A person would be hard
pressed to know all of them in any intimate way. In my opinion it be
unethical for even the most expert old-time Japanese maple grower or
even J.D. Vertrees himself to give a 100% guaranteed ID on any Japanese
maple.

Sweet Little JM.The Text above posted by Gardengal really explains things clearly.A Lot of Garden Centres now on the Japanese Maple Tree Cards have the word "Mixed" assuming both the Variety and sometimes the Origins of the Tree are unidentifiable.Just feel like sitting on that Bench with a Good Gardening Book & a Latte...

Enjoy your Lovely Japanese Maple

Stephen,Cheshire.UK

It looks like suminagashi to me. There's some vids on youtube of that. One with quite a close look at the leaf.

I have a couple grown from seeds that look like it. ;-)

Without a nursery tag, nobody knows for sure....and sometimes that's suspect.

Mike