These take place of the printed XZ plates and provide a more rigid connection between the bearings and YZ plate for a slight gain in strength, most noticeable at the top of the Z travel.
Price for a set of 2.
Aluminum has an "as cut" finish, meaning if you want fancy you will need to polish or sand them a bit. 1/4" thick 6061 T6, 400g for the set.
Steel is acid cleaned of mill scale and oiled. Painting or paste wax is recommended to keep rust away. 1/4" thick A36 / A1011 1,200g for the set.
I doubt it will make much difference but I will put it out there like this. If you are prioritizing, Z moves get the AL, XY moves get the Steel.
To use these you will NOT need the printed XZ Plates, but you WILL need both "XZ Leadscrew Stub Right" and XZ Leadscrew Stub" these are not included.