Confused with A1200 audio connectors.

[Castellen]

The RCA jacks contain switches which join the independant left and right audio channels together if a single (mono) RCA cable is plugged into the computer.  Likewise, you'll also measure a short with nothing plugged in.

I've written more detail about it here, the schematic also shows this detail (poorly).

[Castellen]

Good question, I've never seen these particular connectors stocked by any of the electronics wholesalers I deal with.

I doubt Commodore would have had them made especially, so you could try the likes of Digi-Key to see if you can find modern equivalents.
Else I have some 2nd hand ones which I could supply if you needed a small quantity.  Email me if interested.