They’re definitely different than the old USGI type branded as Colt, Okay, or NHMTG.
The Brownell’s magazines have been around for many years. They were, and if the photos are accurate still are, a two piece body construction with a floor plate that bends to be removed. Same concept as a 30 round magazine.
The originals are a one-piece body only spot welded on the back spine. A piece of spring steel is riveted interior of the front spine and has a foot coming off of it to secure the floor plate by snapping down into a slot/hole.
The latter type was discontinued since they stopped making SureFeed (Okay). I hope someone buys the tooling because they’re great magazines. All of Okay’s magazines, in my opinion, had the most crisp stampings and consistent body dimensions.
Maybe they have improved, but as of about a decade ago the Brownell’s magazines of all types were visibly inconsistent construction and noticeably lesser attention to detail.