The s650 isn't made any more. I'm not certain why you're looking for the part number, but perhaps this is something that your sales rep can assist you with?
I'm working with Cisco service support center to register my contract number for the Ironport. Support indicated that the serial number I gave them, which was obtained from the "version" command, is invalid and thus they required the part number to continue the registration process.
#1 I'd like to find out why the serial number is invalid.
#2 Register the contract number to my CCO so I may obtain support.
I highly recommend calling into our support group and working with a CSE to have this resolved. I believe you may already have done this. This will be the best way to have this resolved.
Thank you for your patience while we sort this issue out.