@lewm those Benz models use a Ruby plate to wrap the coils around. Most cartridges use a ferrous material which increases the output level for any given amount of coil windings, but also (supposedly) causes the assembly to disrupt the magnetic flux when in motion. The downside of the Benz Ruby approach is that it requires much more coil windings to create the same output level (compare to their highest iron cross model Ebony L with 0.26 mV output from 5 ohm coils, or Zebrawood L at 0.4 mV from 12 ohms). But it can also use a much stronger magnet because the armature is not reactive to it. Look at the Gullwing (Ruby) magnet in pics, it’s twice the size of the Glider (iron cross) with the same open body design.
There is an interesting video with Mr Fremer and the Windfeld Ti designer where he briefly mentions the choice of armature material that balances between the two - only slightly ferrous. The Anna uses a completely non-ferrous armature, but not Ruby (polymer?).
Some time ago I too learned you have to be mindful of MC cartridges on some metal platter :)