Specifics of model of piano I think are possibly the least important. I know I'm saying this on Modartt Pianoteq forum but the predominant sound is that of flanger and synthesizers (and production techniques). According to a quick Google search the piano is doubled with synth. That would make it further more difficult to pin it down to a specific piano sound. Either the U4 or the CP80 sound with flanger combined with multiple Moog (choose your emulator's)
With the compression and equalization playing at least equal parts to the actual instruments used.
I think I would be playing with the CP80 sound and see how that splices with the U4 sound. Getting the flanger and synthesizers to blend being far more important to replicating that sound. Equalisation will play a huge part. Getting it bright enough whilst remaining easy on my ear or smooth being the difficult part.
Without looking too closely I think this track is possibly too early for the magnificent GX1 synthesizer. I could be wrong. I didn't Google that part.
"Key Synthesizers & Effects for "SOS":
Minimoog: The core of the famous synth melody, recorded multiple times (quadruple-tracked).
Piano: Doubled with the synth, adding richness.
MXR Flanger: Used on the second piano overdub to create a swirling, distinctive texture.
Other ABBA Synths (around that era):
Yamaha GX-1: A powerful, rare analog synth Benny Andersson used for many hits, though maybe not "SOS" specifically.
Moog Polymoog: Another polyphonic synth in their arsenal.
ARP Odyssey / Avatar: ARP synthesizers were also part of their setup."