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?

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Welcome to the wacky world o' DRAMs. At the time of the 64kb generation (the time of the 4164/4264), there were relatively few variants. That quickly changed, however. NMOS gave way to CMOS ( the "C" in 41256C), page-mode was supplemented by EDO, access times headed south, and variations on allowed addressing modes proliferated like kudzu.

For the 128K, 512K and Plus, all you want is good old, plain-Jane page-mode DRAM. NMOS or CMOS would be fine, provided that the access times are compatible. EDO chips will not work in any of those. That limitation likely applies to the SE, SE/30 and Classic I/II, but I have not tried EDO memory in those models.

Stick with pure page-mode chips, and you will be fine.