The file rks8es.pa indicates it is a version of x8-dhrka, but it is
different.  Presumably, it is for the RKS8E instead of the RK8E.

The RKS8E modifies the drive select from the RK8E.  The RK8E pulls a
single drive select at a time, selecting a single drive.  The RKS8E
encodes the drive lines, allowing four drives instead of the usual two.