"Substantially Identical" was ruled unconstitutionally vague in a 1996 lawsuit. The court ordered the then AG to define what "Substantially Identical" means, Peter Venierio - the then AG - plagiarized the federal AWB's "evil features" list to define substantially identical, which is how this became the standard.
All the M1 Carbine clones are all illegal because AG Paula Dow unilaterally decreed them to be illegal. There was no legal basis for this, only an anti gun AG saying so. IMO this is ripe for a lawsuit, any got the cash to spare for one?