This is all very interesting... but the fact remains that a CAR stock, even fully collapsed, is still not illegal in length... the only thing that makes it illegal is under the AW ban this stock is "telescoping". And if it is pinned, it is not telescoping, collapsable, or folding.
So, if it is legal to pin the car buttstock on a full length buffer tube, there is no logical reason for it to be illegal if pinned to the shorter length on a CAR buffer tube.
There are stocks that have adjustable ends, some of the high tech race gun parts. These "telescope", but are not illegal? Show me the logic.
What we have is some guy at ATF making up laws as he thinks it should be, to be as restrictive as possible on the "bad guns" they want to take from us. The real purpose is a form of terrorism, to scare you out of taking your 2nd Amendment rights. The fines (everything is 10 years/$25,000) are way out of proportion to the offenses. And to make it even worse, ATF is a tax collection agency. There is no tax on post ban "assault weapons". They have no legal standing in enforcing any of the AW laws.
That was discussed extensively when the 1935 Firearms Act was being debated. It was said that restrictions in the bill were unconstitutional. They said no, it was OK because the NFA guns were merely being taxed, not banned.
So... "assault weapons" (that is, guns with 2 or more of the evil features) have no tax on them. Try calling up the ATF and asking for the forms required to pay a return on a post ban assault weapon. If there is a tax, you should be allowed to pay the tax.