There were two versions of the CDI as pictured. Originally with the CDM4 mechanism, and latterly with the CDM9 Pro. Of the CDM4 versions the early ones used a Philips servo board hacked to work with the microswitch in the lid, and later ones used a Naim-designed version. The Naim version sounds a little more analogue in presentation.
The CD2 is a re-engineered CDI to use the front loading tray as seen in the CD3. As Martin says they changed the power supply a little, for the better. The board revision of the CD2 marks it as a CDI v2 out of interest. The CD2 used the CDM9/44 mechanism rather than the pro as used in the CDI. I haven't heard much difference between them and lasers are now scarce compared to the almost bullet-proof CDM4 used in the original.
The CDS, CDI, CD2 and CD3 use the same 16 bit 4x oversampling TDA1541A/SAA7220P/B chipset and same spec of DACs, usually single or double crown. Lots of tweaks are available to get even better performance out of these chipsets. My current favourite is off-boarding the analogue supply to my CDI, but we can shoe-horn an extra regulator board in it instead. That gets me very close to CDS sound.