You can "position" sounds in the stereo field. When you listen to music and a particular instrument seems to be left, it may be the result of microphone positioning, or pan adjustment on the mixing console (whether real or virtual).
Setting an instrument fully left or right creates an awkward feeling, because in real life even when you listen to an instrument played to your left or right, acoustics in the room cause your other ear to hear a delayed (and reflected) version of the sound. Your brain processes the sound and "knows" its position. The absence of sound from that instrument on the opposite ear is unnatural, unless the sound was so quiet that you can barely hear it in the first place.
When setting the pan for an instrument, try to think of it as a position in an arc around the listener (rather than simply "left" or "right"). All instruments set to center will feel as though they are overlapping, but instruments set too far right or left will start to invoke the unnatural lack of sound on the opposite side.
If your mix is dry, that can also explain why slight panning doesn't sound quite as you'd like. Adding reverb to the final mix will make the separate tracks or instruments "blend" better, and sound as though they're in the same room. An instrument panned left will have acoustic reflections added to the right channel by reverb, and improve the perception that the instrument really is positioned where it's intended to be.
Per comments and the other answer(s) I attempted to provide enough information that the OP could arrive at a similar result to the track in question. I am not familiar with the equipment or techniques used in the '60s, so I was working from the position of what I would do in a modern DAW based on what I heard.
That said, I'll leave my answer here because I believe it is still accurate in the respect that the portion of audio heard on the "opposite" channel from the intended position of the instrument comes from acoustic reflections and other artifacts, rather than "hard panning" alone.