After I got an approval letter for permanent residency, I registered myself with the selective service. Then, several days later I went to the INS office to get a temporary I-551 stamp, and INS staff asked me if I wanted to be registered with the selective service. I said "no, thanks, I already did it myelf", but after I left, they still registered me anyway! 
It took me several hours on the phone to clear the misunderstanding with the selective service and remove my second registration number. Ironically, what was my big inconvenience, turned out to be a big help for the OP 
It took me several hours on the phone to clear the misunderstanding with the selective service and remove my second registration number. Ironically, what was my big inconvenience, turned out to be a big help for the OP