Old flows seem to have the number in tiny stamped print near the hanger bracket. Newer flows have it etched, like Matt said. You might have to do some cleaning to find a stamped number (I never found mine till I had them off the car).
I dont know. I am running the Super 40's with an o/r X pipe and really like the sound. They actully seem to drone LESS than my old Dynomax Super Turbos and sound better at idle.