2.1 Front - Overview
This card skimmer is designed to adhere to the front of the ATM machine and skim the user's ATM debit card as it passes through to the real ATM card reader.
In the picture you can see a bit of the material they used to stick the reader to the ATM. It feels like a light-duty caulk. It doesn't have much sticking power. The device came off with just the slightest tug.
Notice that it's shaped just like the face of the card reader on the ATM machine.
Notice how the size and shape of the skimmer body matches that of the real reader on the ATM (image 1.2).
You can read more about discovering it here.