It starts with ERS300A, which is a 2x 2 GB kit that does 1333 MHz with 9-9-9-24 timings, strictly adhering to the JEDEC PC3-10700H specification. Next is the ERS301A, which is a 2x 4 GB kit that also sticks to the JEDEC specification, doing 1333 MHz @ 9-9-9-24.
Things become faster with ERS302A, which is a 2x 2 GB kit carrying XMP-1600 profile, doing 1600 MHz at 9-9-9-24. The ERS303A is a 2x 4 GB version of the same. Lastly, there's ERS304A, which carries an XMP profile that runs the modules at 1333 MHz, albeit with 7-10-10-24, with CAS taking a kit on every other vital latency parameter.
Source: TPU