very interesting question that I wanted to ask during my interviews. Unfortunately It is a sensitive information that they cannot share with you until you become an employee. Normally we cannot talk about the customers for which we are working.
Actually you can talk with some friends inside the company and ask what are the strongest sectors for which the comapny work. Considering that MBBs work for all the biggest companies of the country of reference, you can understand what are the main customers.
Just an example, Bain Italy is the leader in the luxory sector, so what are its main customers? ;)
I hope it helps, for further questions do not hesitate to contact me in private