I can say with almost 100% certainty you will never know for sure what these are with hundreds of named cultivars and many similar green varieties ...forget it ..plus if they are true non grafted seedlings it is irrelevant since seedling are NOT necessarily true to the mother plant and it would be improper to pass 'em off as such...other than " seedling plants of ??? cultivar ..give them a name yourself and be done with it...David

Great advice, thank you. It's not that I really care about a name....just wondering what size they may get so I know where to plant them.
Thanks again.

Possibly seedlings of Seiryu that will not be true from seed.

Being seedlings strictly speaking they are just Acer palmatum. Period. However, if the one on the left turns out later to be indistinguishable from a named form such as 'Seiryu' then it might be considered legit to call it such. It depends on if the previously existing named cultivar (like 'Seiryu') is really still just one clone (single seedling or sport reproduced by grafting or cuttings so that every specimen has been grown from a fragment of this one genetic individual) or if it has become a multiclonal cultivar (Cultivar Group) represented by more than one independent introduction. Tree cultivars tend to be clonal but not all are. And annual flower and vegetable cultivars raised from seed year after year never are.

Acer palmatum 'Atropurpureum' is now often listed as A. palmatum var. atropurpureum because thousands of purpleleaf seedlings of varying characteristics have been raised and sold as 'Atropurpureum'. (VARietas, being a botanical term is supposed to be limited to variants occurring in the wild, so I wonder if using this to cover purpleleaf seedlings occurring in cultivation is correct--it would depend on if purpleleaf trees also pop up in the woods of eastern Asia).