First off, is there any regulation on them in Illinois other than typical firearm regulations?  

Also, is there a difference between rifle and pistol lowers as far as paperwork, etc?  I noticed in an email from PSA they listed Complete MOE pistol lower receivers.  Is that simply saying they are set up for pistol w/o a stock?  

Mine were all bought as stripped lowers and I don't remember declaring them either way or being listed either way.

Stripped should have been marked "other" which can be built either way. But if you build a rifle first technically it can't ever become a pistol. Build a pistol first you can go back and forth.

An FFL told me an acceptable alternative to 'other' is 'lower' because the BATF auditors won't confuse it with 'any other weapon' ... apparently as long as it doesn't specify 'rifle' you are legit to build as desired (see quoted above about rifle -> pistol is naughty) ... but who knows.

In another thread I posted a link to the explanation for this box on the 4437. Will try to find it and add here