Yup, I have 2 Stribogs (SPa1 Gen 2, and SPa3) plus two Scorpions (4” barrel mini and 7+” barrel), all with braces. Between the 4 of them, I have several of each type on mags, but this lower receiver does look interesting. The serial number on my one Stribog appears in several places: 1) on the bolt viewed through the ejection port, 2) under the ejection port on upper receiver, 3) lower receiver above the pistol grip, and 4), on lower receiver above the trigger. I’m looking at my SPa1 Gen 2, which was a special build making a SPa1 into a SPa1K short version, So I’m not sure if that altered anything concerning what’s considered the true serialized part of the firearm for ATF purposes. My full-size SPa3 isn’t handy at the moment, so I’ll check that later. I’d venture to say that the linked company probably knows what part is considered the ”true” serialization, so it’s probably legit.