Some 256Kx1 DRAMs that can be looked up at
http://www.datasheets.org.uk/ appear at the end of this post. The trailing -NN access time speed designator should not be included in the part number lookup as its meaning will appear as an explanatory note within the base part number datasheet. The CMOS or NMOS designation is just a technology comment and should likewise not be put into the part number search field.
There are parts faster than the examples shown below. To look up a part datasheet requires knowledge of the part number, which may only be partially indicated on the part marking. The part marking often includes the mfg logo and some trailing digits, omitting the all important prefix letters and numbers. Faced with such a chip, you have to guess the mfg then from the examples that you know, add prefixes to try a search.
Given
4256 try SMJ4256
41256-15 try uPD41256, MSM41256, or KM41256 (one at a time!)
81257-15 try MB81257
41256C try uPD41256, then see if a C datasheet appears as a separate choice.
I leave it to interested readers to verify that page mode compatibility, or whatever, is met or not met by inspecting the datasheet of the parts at hand. The parts listed below are just examples of DRAM part numbers in use by leading manufacturers at that time. It's not a complete list, and most certainly does not imply fitness for any particular use.
100 NS
Hyundai HY53C256L-10 CMOS
Samsung KM41C256-10 CMOS, KM41C257-10 CMOS, KM41C258-10 CMOS
OKI MSM41256A-10 NMOS, MSM41257A-10 NMOS
Fujitsu MB81256-10 CMOS
120 NS
Hitachi HM51258-12 CMOS
Fujitsu MB81256-12 CMOS
NEC uPD41256-12 NMOS, uPD41257-12 NMOS
OKI MSM41256A-12 NMOS, MSM41257A-12 NMOS
Samsung KM41256A-12 NMOS
150 NS
Hitachi HM51258-15 CMOS, HM50256-15 NMOS
NEC uPD41257-15 NMOS
OKI MSM41256A-15 NMOS, MSM41257A-15 NMOS
TI SMJ4256-15 NMOS
Samsung KM41256A-15 NMOS
200 NS
Hitachi HM50256-20 NMOS
TI SMJ4256-20 NMOS