Greg - the residue is indeed ferrous/magnetic - a magnet is fitted on some models of the 722-6 - tried a magnet on my filter (split open) last time and patterns emerged when used underneath.(that's a finger smear in the pic)
As you note, the magnet wasn't fitted on all models, and those who have the magnet in the pan have indicated that it has come out virtually devoid of any metal, so it looks like the filter media is mostly doing it's job and the vast majority of the detritus is non-ferrous. Patterns in the filter are really de minimis
; Did you run the magnet through the pan? Moreover, the special MB fluid is designed to suspend particles (which was part of the whole "lifetime" nonsense) and most of the moving parts are aluminum, stainless and various other non-ferrous compounds (have you seen the youtube of it's 7-speed progeny being assembled?) so unless there is a serious problem with the transmission, I'd be surprised to find metal of any significance on the magnet (and if there is a serious problem, a magnet isn't going to be much help
). Either way, as noted, it's a cheap part.
I started playing with auto transmissions back in the 70s (mostly on 60s vintage stuff) and back then, you opened the pan and the magnet would be covered in bits of stuff; you couldn't even see the magnet for the crap covering it. And that was just normal as the bits shed from the clutch material (some plates were metal-impregnated), gears, bushings, etc., there was lots of ferrous stuff to accumulate. One TorqueFlite box I opened had about 1/3 of an inch of goo in the bottom of the pan, it had about 90K miles on it, I was kind of surprised that the fluid change didn't result in it failing shortly thereafter.
At any rate, that's why I don't get too worked up about the lack of a magnet in the 722.6, mine had one but the fluid was pretty shot and there was nothing of significance really in the pan or on the magnet. The filter wasn't broken open so I can't say anything about that. (I had mine done at a shop nearby, spoke both with the manager and the tech that worked on my car, both knowledgeable and helpful guys.) Of course it had only about 72,000 miles, so I didn't expect it to be really bad, I was just impressed with how smoothly it shifted afterward.
Take care and enjoy the ride,