It's filter switch. There's a cap wired to it. Looks factory to me.
That makes sense! I didn't make that connection to a bass because I thought the HB-1s were original on that harness. But yes, looks like a JS 2 bass harness. This below has 1973 and 1974 pots:I believe that it originally may have been a harness for a bass with two HB-1s attached to it!