It has been my experience in the past, from reviewed Material Certs, that it does. However, if possible, if it cannot be clarified on the cert, then contacting the supplier/manufacturer should help you to ensure that designation.
Also, I did a quick scan of ASME SA-20 which is the standard specification for General Requirements fo Steel Plates for Pressure Vessels. (It is referenced in ASME SA-516.) I didn't see the 'N' designation. (Again ... QUICK SCAN.) You may want to review that document also.
The appropriate designator to be marked on the plate is "MT" per SA-20 for heat treatment, which is defined by SA-516 as normalizing when heat treatment is performed. The N designation was commonly used by manufacturers on their MTR's and engineers in their specifications to designate that the plate had been normalized or required a normalizing heat treatment, respectively.
