It's not a National or Linear Technology part. They both have package code to device cross references, and FA51 does not show up on either list. Unfortunately, other vendors don't provide such information, at least not where I could find it.
You may have to trace the circuit out, to get an idea of what it might be. In your picture, pin 1 is the lower left corner, and the numbers go CCW around the chip. For a positive regulator in this package, pin 1 is usually Vin, Pin 2 is ground, and pin 5 is Vout. Pin three is usually an On/Off pin, and pin 4 is a bypass pin for low noise output.
Edit: Looking at the photo more closely, I'm not so sure that it is a voltage regulator. The inductor (L4) in the photo is between pin 1 and pin 5. If it were a voltage regulator, this would be tying Vin and Vout together, since inductors are a DC short.