Hey Erok,
Let me put my limited knowledge two cents in here. The 5900 Pro SL is basically the exact same machine as the Blue and Gray. The 5900 has a meter that shows more common coin type targets like dime,quarter,gold,etc on the meter face.The meter face is yellow-black. The Blue and Gray shows relic hunting specific targets on the meter faceplate such as lead,caliber bullet(44,58,69) among other things like large/small brass. This meter is mainly black-red. The meter is the same, just the faceplate is different. The only other difference is that the Blue and Gray has different stickers on the side of the box. Contrary to popular belief, there are no other differences(internal or external) if you compare the same model year 5900 SL Pro to a Blue and Gray. I have heard that some early 5900's and Blue and Gray's came with a volume control, but I have yet to see one. I think this belief may come from the older non-SL version 5900's that were in the older blue box, standard handle version. These had volume controls on them. White's may have put some of the earlier circuit boards in the first 5900 Pro SLs produced. The last version/type of 5900/B&G had the "CB" designation added after the serial number and it meant "composite board". Most people would agree that there were very few differences in the 5900/B&G Sl versions-even the CB version. They are all deep seeking and work well in extreme soil. I have one of each myself. If you find a 5900 and want to convert it to a Blue and Gray, let me know. I have the B&G faceplate and side decals NOS. I hope this info helps. I am sure someone else out on the board may know more as well but most of my info on these has come straight from the factory.