It looks to me like they started with a Pacesetter knockoff header from ebay, used the primary pipes and flange but made their own crossover, rear collector and downpipe. Also rewelded on the outside of the primary to flange and ground the inside welds.
Not a bad way of doing it but I doubt that Mike will be able to rectify some of the issues that Bill is highlighting because they don't make that part of the header.
As for the O2 sensor in the crossover, not been an issue in my experience, and in fact can benefit cams with a lot of overlap that create exhaust reversion because the O2 is further away from the head ports compared to reading from the collector.
