NEC (MT) - 1 Megabit

How do I read the type number?

421000AA64FB-60
^^----------------- 42 = NEC DRAM
421000AA64FB-60
  ^^^^------------- Internal Organization & Refresh: 
                    1000 = 1Meg x 1  / ??  refresh
                    4256 = 256K x 4  / ??  refresh
421000AA64FB-60
             ^^---- Speed: 50=50ns / 60=60ns / 70=70ns / etc