That's what I'm thinking: It was for some stability improvement or to prevent excess current flow and overload recovery issues when the input CM range was exceeded.The 1k may be some obscure stability improvement (I vaguely recall some opamp specific tricks like that.. the R values look wrong for DC correction. The effect of source R on stability is not simple.
And that would generally make it wrong for impedance-balancing the input and feedback impedances for THD improvement.the R values look wrong for DC correction.
I think I managed to get some THD improvement out of an OPA2604 using a 1K in the input.
But the OPA2604 is a BIFET and the JRC4580 PNP BJT.
I liked the 4560s. Otari used those a lot too and they're a frequent staple in Soundcards.