I have followed this thread since before I got my M2C.
There were no marks on the gauges when I took delivery.
Several weeks later, while detailing my car, I noticed dried droplets of some kind on the gauges.
Very carefully, I used a brand new, soft microfiber cloth (damp with water only) to very gently wipe the foreign substance off the gauge surface.
I held my breath and used no pressure from my hand.
Just the weight of the microfiber cloth brushed the gauge surface clean.
I then used another brand new dry microfiber cloth to gently touch the gauge surface dry.
Luckily, no damage was done.
Whew.
I hope I don’t have to do it again.