A piece of sheet metal, bent and welded into shape, makes a strong, reliable BB shell. It may not be pretty, but it is serviceable. In low end frames, the tubes are brazed in with brass, and in high end frames, it's the same, but the tubes are mitered before brazing. Yours looks to be brazed according to a high end practice. Yes, the threads are applied after the tubes are brazed into the shell. So I would say your frame is neither low end or top end, but it is of high quality.