though the part numbers are weird, they should all just read same as a generic 27 series chip. find yourself any manufacture 27C64, 27C256 or whatever part numbers they say they are... the number after the C is the size and just match it up to the number after the 27. 27C64,128,256,512 for under 1 meg chips... or numbers like 27C010, 040 080 for over 1 meg sized chips.
so a 2764 turns to 27C64
and a 27256 a 27C256
same with the weird Hitachi part numbers... 4827128-25 becomes 27C128 the -25 is the rated speed which is not important here unless you are swapping them out.
just keep an eye out for binary multiples1,2,4,8,16,32,64,128,256,512. and slot them into your 27C series part numbers.