That's my guess. I would assume the parts are over molded. Would make the most sense.The part that needs milled out is formed first then the rest of the polymer lower is formed around it. They don't mill it out and fill it back in, it's a very clear distinction. Ares also doesn't complete the lowers and sell them without the serial. They sell you the blank and have the equipment there to finish it yourself. You must do all the work yourself.
Well, that's about right, but I'm starting to believe that EP Armory tried (probably with the best of intentions) to make things a little easy and push the "non-firearm" definition as far as they could. Don't know, but that's just what it look like to me.
I’ve been doing a little looking about where the dividing line between “non-firearm” and “firearm” is when it comes to partially finished lowers. I have not found any definitive answer, but the best way for a manufacturer to avoid problems is to submit the part and get a ruling from the ATF. Did EP Armory do this? I don’t know.
What I have found is a letter from the ATF to another manufacturer which lays out some of the rules and explains why their proposed “80% lower” is a firearm, not a “non-firearm”.
http://www.quentindefense.com/downloads/ATF Letter Non Gun.pdf
The most important thing is that the “hammer/trigger recess” must be solid. It does not say filled in or molded first or any such thing. It says “solid”. It must be “completely solid and unmachined in the trigger/hammer recess area.” Does something that is not made of the same material, fills the cativity, and can be pulled out after some drilling make for a “solid” receiver? In my mind (the only one I have) “solid” means that the “trigger/hammer recess area” has to be a homogeneous material, not a different material (I mean "different" as in molded at a different time such that it is not homogeneous). I know that there has been talk about what was molded first, inner or outer, but if the standard is “solid”, that would not seem to matter. It may address one issue, avoiding the lower being manufactured as a firearm then “converted”, but it does not address the other issue, the lower not being truly “solid”.
Listen to what he says at the beginning of the video, and then at about the 6:05 mark and on.
https://www.youtube.com/watch?v=9fSEodxoQ5Y
On this one, go to 5:00 and watch. That’s from a “solid” “trigger/hammer recess area”?
https://www.youtube.com/watch?v=XDmFUz-ceVM
Also, in the letter linked above, it specifically rejects the design because the selector hole was present. Once again, the letter said the area had to be “solid”. The EP Armory lowers have guidance “dimples” for the selector switch and fire control group. Is that solid? Maybe, maybe not, but the ATF seemed pretty clear in the letter that they didn’t want holes in those area for a “non-firearm”. I don’t even know if that’s the issue. I would think the “solid” thing was more of a concern.