What's the difference between 4256, 41256-15 and 81257-15 RAM chips?

Do additional letters and numbers make a significant difference (e.g. 41256C-15)?

256 seems to be the main commonality between the chips, but 257 throws me.

All are used in 512K boards.

Can 128K RAM chips be identified by anything other than 4264, are there just as many variations?