The ordering ID numbers and marking ID numbers are different for the same PROM device, so the designation you use to order a part will not match the marking on the actual part.
Marking ID numbers exist because the complete PROM ordering ID cannot fit on the small PROM packages and still be readable. (A Marking ID is basically an abbreviated version of the Ordering ID.)
Please refer to the PROM section in the latest release of the device data sheet for an explanation of the Ordering ID and Marking ID numbers for PROMs:
or see (Xilinx Answer 1067).