This is one of the common repairs for an upright piano action. As the hammer head, stem, and butt assembly returns after striking the strings, the hammer butt is caught by the back check attached to the whippen assembly. The back check contacts a small stem and buckskin covered wooden plate attached to the rear of the hammer butt.
This creates a contact point that has considerable wear; in a piano action where the regulation is neglected, or left out of proper mechanical adjustment, the wear on this point is considerable over time. Once this small piece of buckskin is worn through the wear begins on the original, obsolete wooden parts.
Obsolete piano action parts are difficult to obtain. At one time there were so many different piano manufacturers, it is impossible for the piano parts makers to supply parts with the correct geometric measurements for every make and model of piano produced. The parts suppliers have decided to make generic fit parts. These parts come close to the original and will work properly; sometimes with geometric adjustments to the parts or the action itself.
There is of course, the usual wear that happens with regular use of the piano. This is why it is so important for the older pianos to be kept in proper mechanical adjustment. This is called regulation of the piano action. When the piano action is kept is proper regulation and the felts and buckskin within the action are kept in good condition, very little damage is done to the original wooden parts.
Here is a photo album of the replacement of the hammer butt shoulder buckskin. There is text to read below each frame in the photos.