Altera EPLD EP1810GC-35

Hello, I am working with the United States Gov't on an obsolete component for one of their systems. The part number needed is an EP1810GC-35. The part that I can supply is an EP1810GC-35AB. I have been searching high and low and cannot find what the meaning of the "AB" is. Can anyone help me with this?

Thank you in advance.

-Jeremy