I don't know why people would want to know the serial numbers on a blue gray except to ID or date them. As far as I know the Blue Grays are all the same. There are some differences in some of the other series of whites though. The 6000 for one.
The Blue Gray is kind of a cross between the 5900 and the 6000.It was set up for relic hunting in that some of the knobs were moved and the meter set up for relics instead of coins. I still have my old 5900 and the Blue Gray is a much better machine. the Blue Gray and 6000 both are great machines and both work well for coins and jewelry. I think they are 2 of the best all around machines Whites ever made. I still use my old Blue Gray.