MIPS I-IV instruction set standards

[Angelo Papenhoff]

I'm wondering where the MIPS I-IV standards that are referenced
everywhere are defined. I was able to actually find what seems to be the
IV standard [1] but found no such thing for I-III. I didn't even find
any bibliographic references to them. Did they only exist as printed
books and nobody bothered to scan them? Or are they under copyright?
Would be nice to have them accessible somewhere.

[1]
http://www.cs.cmu.edu/afs/cs/academic/class/15740-f97/public/doc/mips-isa.pdf