Sony's 1st consumer & audiophile player, sporting a Digital output as well (ES-D). And that output was way ahead of its time - or way behind :

Given the "image-on-disc" ascendency of the CD format (the latter being only the side-result of the former) and its early embodiment with a VCR + PCM unit to store the 0s and 1s, it was planned to use CD as a "graphics" source as well. JVC's VHD and AHD format shared with CD the same planned "exchanges" between audio and video, btw.

However, VHD/AHD never really made it and formats do evolve - even the winning ones like CD. So, "image-on-disc" finally went to Laservision exclusively, before CD-i came in, before Laserdisc's CD-V and later LD-G[raphics] versions failed to suceed and before DVD took on the role of Attila the Hun...

But the thought was planted way before all this and the 552ESD bore a little of it, right from way back in the late 60s, when Philips planned a two-headed format: same disc-size (30cm), same players but different content...

That reminiscence is the little back switch allowing to output either a PCM digital audio stream or a "graphics" data stream. However, that I'm aware of, no red-book CD ever had graphics on it and I've never seen anything capable of being hooked to that output either, just like the "accessory" plug found on all of Sony's 1st generation CD players...

Anyway, back to the topic of this post :-)
Although Sony used its own DACs from the CDP-101 onward, the 552ESD was the first high-end unit using other makes, a (fairly strange) move which stopped with the introduction of the CDP-X77ES in 1990 and its Sony/NTT PULSE chips.
Sony's 1st outboard D/A units (DAS-702 and DAS-703ES) used Sony's own DACs, while the 1987 DAS-R1 reference used... Silver Crown Philips' TDA-1541 !