These are the initials of Stephen Proctor who is a former owner of the Fountains Estate. These initials are on the front of Fountains Hall and are accompanied with an otter clasping a fish in it's mouth. Fountains Hall was built between 1598 and 1604 by Stephen Proctor, in the style of Elizabethan architect Robert Smythson.
The Hall itself is actually built of sandstone, taken from Fountains Abbey itself. However, the outer facade, windows and statues of the Hall are made from fresh limestone, brought in from outside of the estate.