This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
CDS615E (2x2/1MB;SCSI, based on the Sony CDU920S)
CDS620E (4x2/1MB;SCSI, based on the Sony CDU924S)
CD-R2x6 (6x2/512K;SCSI, based on the Sony CDU926S??)
The CD-R2 is the CDS615E in an external case. The CD-R2x4 might be the
external version of the CDS620E. The CD-R2x6 probably has a name like
CDS640E, but it's not listed as such on their web site.