The receiver extension is definitely sized for "Mil" spec. Commercial spec is just a bit larger (thicker). If you end up with a good deal with a commercial one, they will fit pretty well with some electrical tape, etc on the tube. Not the greatest option but works well if you have a commercial laying around or get one cheap. I had an Magpul STR in commercial and it fits great with the above mentioned mod. Fit fine without the tape but much snugger with a layer of elec tape added.
Good thing is Mil spec versions are usually easier to find and come in more models. So if ordering new, then just get the Mil spec version. But if a friend has a hand me down commercial, it can be made to work just fine!
Good luck