Complete the "You've Gotta Shoot Them In The Head" mission given by Mr. Crowley in Underworld. Return to him once you have all four keys. Follow him out of Underworld and take back the keys. Go to "Fort Constantine", located in the upper-left corner of the map directly between and slightly to the north of "SATCOM Array NW-05a" and "SATCOM Array NN-03d". Enter the little house on the left named the "CO Quarters". Proceed downstairs and use the key on the "launch control bunker" door. Be prepared to fend off robots while making your way further down. Eventually you will find doors for all of the keys, the last one being a room that holds the T-51b under glass. Use the computer to free your prize.