It was five piano notes played at the same time by producer George Martin. This Wired article has the details.
It again shows the power of combining multiple instruments together, to a point where the listeners have wondered about this beginning chord for a long time.
With MIDI information it’s a no-brainer to combine various instruments. Long time ago when synths were expensive I did similar things with the same MIDI track used on multiple tracks with different Roland Jupiter-6 sounds, resulting in very complex soundscapes and chord sounds.