The Bell Thogchag is the sixty-ninth treasure that can be found in Uncharted 2: Among Thieves.
The Bell Thogchag is found in Chapter 19 - Siege. About halfway down the slope, you'll see a small stone structure off of the main wooden pathway. This stone structure has a staircase leading up to it, and is surrounded by some aged wood, colored both red and blue. The treasure sits within this small wooden structure, and is rather easy to get to. Just be sure not to neglect grabbing this treasure en route down the slope, because the never ending firefight ensuing around you could prove to be quite the distraction.