I questioned same part # discepancy and it was explained to me that the 8603 number was the hub/pulley only which is indeed shown as obsolete by most all sources I found.
I was told the 8604 number was for the complete assembly (which I'm sure is a reproduction and not an NOS).
Regardless, for my purposes anyway, if it fits and works I'll be satisfied. If it doesn't I'll keep your info in mind. Thanks for the tip.