If I may, I would offer two points:
1. It is always helpful to specify the origin of the schematic being referenced. In this case, the cap in question is in fact C9 -- if you're looking at the Sams schematic, but it's C28 on an actual Mac schematic for the MC240. For continuity of discussion, my references here continue on with the Sams component designations.
2. As for the purpose and why there is only one -- that's because there are in fact two -- one for each channel -- but one is hidden. In the right channel, that cap takes the form of the shielded cable used to connect the output from the section of V1 supplying the right channel signal (pin 6) to V7, which is physically located some distance from V1. For the left channel, the output of V1 (pin 1) goes directly next door to V2, so no shielded cable is required for such a short connection. The cap is therefore not so much a necessary ingredient for any specific suppression or filtering function, as it is an effort to make the circuit conditions identical between the two channels, in spite of differing physical layout/lead dress conditions between them. If you look closely at the schematic then, the "other" cap is shown as the shielded cable running from pin 6 of V1 to V7 for the right channel, where the physical cap C9 is used to replicate the capacitance of that cable in the left channel.
Gotta love Mac's attention to detail!
I hope this helps!
Dave
