That nut was holding the cartridge together, and you should not have removed it. You do need to remove the sleeve to access the bonnet nut which hold it all together. WHen you get to that, DO NOT overtorque it. If it does not turn, you want to use heat, or split the bonnet nut with a hacksaw or dremel.
I have a diagram of the 1500 which I will scan in and post later this evening.