Receiver Extension Tube 101
The threads for the tube are the same in all lowers (or should be anyway). The tubes for telscoping stocks are different, the milspec tubes are 1.150 iirc and the aftermarket tubes run all over the place, but are generally around 1.175 or so...
When Dick Drasen was making the aftermarket copy, he used this slightly oversize tube and it has become the de facto standard in the industry. The reason is mostly the production technique and associated cost -- a milspec tube has the threads cut and then the body is reduced by "shaving" or planing along the length of the tube with a special cutter... you can get an idea of how this is done by looking at an LMT tube, the cuts are made in three passes and the LMT process does not blend each cut perfectly, so you have those two "ridges" the run the length of the tube at about 2 and 11
The aftermarket tubes do not reduce the size of the body... but this does NOT mean they are stronger, if fact, because the threads on the aftermarket tubes are generally not cut to full profile, they are demonstrably weaker -- this is one of the first places you will see a rifle break.
Rifle tubes are all in the same box... and the Magpull is out by itself.