Gift of the Class of 1915 - Sundial on Old Main Lawn
The class funded the purchase of the sundial that sits in the middle of Old Main lawn instead of purchasing class ivy. The arm of the sundial has been removed after it was stolen or broken off numerous times. After the sundial was removed in 1933 and only the stone pillar remained, the class paid for its restoration in 1950.

Gift of the Class of 1966 - Armillary Sphere
Often mistaken for the Class of 1915’s sundial, which is actually located on Old Main Lawn, the armillary sphere on the terrace of Old Main is actually a gift from the Class of 1966. Armillary spheres were used before the Common Era to measure celestial objects in relation to Earth. The sphere is perched on the back of a turtle, a nod to the Greek god Atlas who held the world on his shoulders.
A common myth held by many Penn Staters is that the sphere marks the exact center of Pennsylvania. The idea is based on simple logic: the sphere is in the center of Pennsylvania’s Land Grant University, in the center of the state, and in Centre County. But it’s not true. According to geographers, the center of Pennsylvania actually sits somewhere between State College and Bellefonte, near the fish hatcheries of Fisherman’s Paradise or on the grounds of the State Correctional Institution at Rockview.

The final coordinates for this letterbox are: N 40° 47.ABC' W 77° 51.DEF'.
Go to the posted coordinates, look around you, and you will find everything you need to figure out the clues:
Checksum: A + B + C + D + E + F = Armillary Sphere's Class - Sundial's Class - "YET"