E-Slim are European style nails. These fit well in shoes like the Kerckhaert DF and SSP series that were made first for the European market. Used most often in the Hunter and Jumper discipline. Some farriers prefer to punch their hand made shoes for E-heads because of the symmetrical nature of the nail. There is also a feeling that an E-head nail has more strength because the shank comes out of the center of the head, with slightly more mass.
