As described on the EQP-1S3 product page, the 2U EQP-1S3 is an enhance version of the EQP-1A3.  The EQM-1S3 (mastering version) takes the EQP-1S3 one step further by replacing the BOOST/ATTEN potentiometers with 21-position (and BW with 11-position) ELMA switches, loaded with precision metal film resistors to achieve custom tapers for each control. 

The current design implements our “full-range, progressive-taper” design, which provides 0.5 dB per step precision for the first 50% of rotation and then progressively larger (1 or 2 dB) steps as the control is rotated from 50% to 100% of rotation.  This allows precise control for subtle equalization for mix bus or similar applications or the full 18-20 dB BOOST/ATTEN of the potted version when tracking or mixing of individual tracks.  Furthermore, the stepped controls make recall a breeze and the precise tracking between units is essential for stereo image retention when using a pair on the stereo mix bus.