I'm digging into a circuit that was designed by someone else and it uses a dual opamp (specifically the JRC4558) but strangely pins 5, 6 and 7 are not used. Supposedly, the JRC4558 has a desirable sound but why use only one side of a dual opamp? Is there no single opamp version of the JRC4558 or are the duals just cost effective? Maybe they had some laying around and used them in the circuit due to availability. The circuit is a Dan Armstrong Orange Squeezer.

Anyone have any insight on this?

It could be as simple as keeping the board layout and components standardized. Buying, designing in, and soldering in a different part has costs. If they used sockets, it means perhaps buying and stocking an 8 pin socket for just that one op amp, whereas if they used the dual op amp then they could use the same larger sockets they use for other parts.

Also, having a spare op amp might have been considered attractive for potential future changes to the circuit. It gives easy access if they want another buffer or a signal inverter.

I think 20pcs for $2 all over teh google probably has something to do with it.

Yeah, I'm sure it wss either an availability or cost issue. Who knows what was happening in the market back in the 80s.