I have a preban upper receiver. I'm considering collapsable butt stock. So do I need a preban lower receiver?

Is there any difference between pre and post ban receivers? Some vendors say there is no difference.  

I want to be sure I order the right lower in order to buy the kind of butt stock I'm considering.

Thanks in advance.

Unless you're in a state with a state AWB, there's no difference betwee  pre- and post-ban lowers.  And if you do live in a state with a state AWB, the only difference is in how the state treats it.  The only difference is in the date of manufacture; they're functionally identically.

What is it physically about the receiver that would make it post ban or pre ban? I thought all the ban stuff was with the upper and the stock only. I've heard there are differnces in sizes of the tubes on the lower. Any help would be appreciated. Thanks!

There's no difference in the lowers.  Colt made some lower receivers with non-standard pin sizes, but that had nothing to do with the ban.  Also, carbine buffer tubes (or receiver extensions, if you prefer) come in two sizes, but that doesn't have anything to do with the ban either.  The threaded part is the same on all of them, but mil-spec tubes are slightly different dimensions than the standard commercial tubes once you get past the threads.  If you want to use a VLTOR or LMT stock, you need a mil-spec buffer tube.  If you buy the complete buttsotck assembly, it will come with the correct tube.  But again, the difference is in the diameter of the tube once you get past the threading.  They're all the same where they screw into the lower.

Well, you are asking a question that might be helped with some additonal facts, such as:

Do you already have a reciever or are you going to buy one

What state are you in

What makes a reciever "pre-ban" is teh date of manufacture.  The difference in sizes in the reciever holes is only applicable to a Colt lower AFAIK.

So assuming you live in a non-ban state, then you can get any mil-spec lower and it should work fine with what you already have as well as the collapsable buttstock.

Date of manufacture has nothing to do with pre or post ban the lower has to have been built into a complete weapon prior to the ban to be legally a pre ban, I live in Massachusetts which has a AWB
I had a thirteen day window from the sunset of the federal ban till the states went into effect so to be legal I modified all my post ban rifles to pre ban took pictures of each and had the notized prior to end of thirteen day window know there legal here.

There is no ban. There is no difference physically in any feature regarding the lower. So consequently for me it does't matter. What matters was availability of lowers. No one, for the last 2 weeks have had stripped lowers except for Les Baer $175 and Fulton Arms ($150) so I went with fulton arms. All these things are about the same aside from logos. The Fulton Arms has some sort of teflon coating and the quality appeared to be top notch.