I've had this issue before. Last year when my work permit was issued, I read that within 7 days after getting my permit, I would be issued my ssn. That didn't happen and like you I had a job just waiting to complete the process. I made an appt with ssa and went to do my application. 2 weeks after my ssn was issued and then 2 weeks after that, another mail came with a different card number. I had already distributed the 1st number to my banks, dmv and my job so when I went to the appt to get it fixed, they did their thing and they said the new number would be 'deleted' however I was given back the new card with an attachment as a just in case for future reference. I kept using the first one I was issued and so far I haven't had any issues.