Receiver extensions and thread fit

I recently picked up an SBA3 pistol brace. As I began assembly, I noticed the castle nut had a very poor thread fit. I then threaded the extension into my lower and it too fit poorly. I then looked into the specification for the receiver extension. While the SBA3 uses Mil spec tube diameter, the threaded portion checks out as a mishmash of specs between the two styles. The major diameter of the threaded area matches the smaller 'Commercial" tube spec (measures 1.170-1.177, yes out of round too), but the threads have well formed peaks as the larger diameter Mil spec requires instead of being truncated as the commercial spec is supposed to. All of this added up to the extension having quite a bit of wiggle. I imagine, with the current trend of everyone gooning on their own AR's the company might have done this on purpose to ensure the extension will fit into damn near anything. I would speculate the company would rather deal with a few people that are not happy with the fit rather than hundreds of people that can't get their extension to thread into their Anderson lower. I'll be looking for a new mil spec receiver extension with a proper thread fit before I put this thing together.
I'm posting here to see if anyone has had a similar experience?