How much current does it draw? That is a dead giveaway. A CMOS chip will draw less than 20ma with about 13ma being typical and an NMOS chip will draw over 70ma with about 100ma being typical.

Looks like it only draws 5.2 mA when running at 1MHz. I just have the data bus tied to a value of EA and ...

It works fine but treats [NMOS] undocumented opcodes as NOPs (such as the various JAMs, LAX, etc.) and it seems to implement the opcodes I'm familiar with from the 65C02 such as BRA, STZ, PHX, PLY, etc. Is this chip actually CMOS and mislabeled? Seems to me you just answered your own question.

(I ...

Interesting. I think it must be remarked since it seems to act like a 65C02 but yeah, I'm not sure why someone would do that. Maybe the old NMOS chips are getting scarce/more expensive? I've used WDC 65C02s which do have those new instructions. I didn't expect this one to have them.

@cjs: "I have a couple of parts with 2010 date codes on them"
Those are fakes (or at least the date is fake). Rockwell stopped existing in 2001.
Perhaps I'm reading the date codes wrong?

I guess it's possible that the R65C02P4 chips in my photo above were relabeled to a faster part number, so as ...