Thank you for being part of the community.
This is a very good question, glad I was able to find an answer.
On the width side 15 3/4" (40cm), on the back of the mirror, you will find 2 reinforcement supports (bars) at 11 3/4" (30cm) from the top and bottom of the mirror.
In those 2 supports, there are 2 holes, one on each side, so 4 holes in total. This will allow you to insert your anchors permitting you to hang the mirror either on the short or the long side depending on your preference.
Found your solution? Simply click on Accept as solution to help others with similar questions!