Considering that Magnus can embody powerful mages when he pleases, it is possible that he forged the staff through one of his disciples.
Also, here are a few things I think are important clues:
- The Eye is found in Saarthal; the supposed first settlement made by men in Tamriel
- The Eye has markings in an unknown language. Neither Man nor Mer can read what is written, and I'm pretty sure Dwarven should be ruled out as well.
- Magnus had a part to play in the creation of the world; Ancano taps into the Eye and exclaims: "The power to unmake the world at my fingertips" or some nonsense like that. Surely if Magnus played such a prominent role in creating the world, whatever artifacts he used also have the power to unmake it as well.
TL;DR - my bet is that the "Magnus" in mention is the God of Magic.