I just had another look at it and I actually think this *might* be a GHL.
(look at my signature, I know how a GHL looks like
My guess is that it came off a non-S Boxster and then someone made his own Boxster S mounting kit.
btw: the fact that the u-tubes which connect the exhaust with the cats are included is a HUGE benefit: when I installed my GHL a year ago it took me FOREVER (and many foul words) to get the original u-tubes into the GHL. Talk about a tight fit...