On Normal you must defeat Omega in N-1, then go past the door which says "I need something to open this door. The Game Pack Megaman Zero 4 is needed" and enter the next one. You'll get Mysterious Rock.

Then, after you defeat Serpent's final form, you should have "COMPLETE" over your save file. Now go talk to Fluub, and he'll say it's Model OX.

Submitted by xemnas
Correction by chaoszera