Actually it depends from coach to coach that whether SMB is present or not.
For example, some days ago, I had a reservation in Mumbai GR in which I had a SMB but after boarding the train I found that my coach don't have any SMB and my SMB seat number turned out to be a SL berth
So.untill and unless u board the train u will not be able to understood whether the coach has 64 or 72 seats