User blog comment:Mr Bio Shock/Attempting to Understand Infinite's Ending... (Spoilers, of course)/@comment-3048593-20130329212731/@comment-69.85.234.252-20130405143251

Comstock knew all along that he was Booker and Booker was him. Hence at the end when he mentions the pinky--because he was there when DeWitt grabbed him and caused her finger to be snapped off.