Been through the armorer class for the S&W 3rd gen models 4 times. Carried some issued ones since '89/'90 and own some more. Never heard of a factory "fiber buffer" for them.
FWIW, the fiber insert used in the Sigma trigger spring, and now in the M&P triggers, are due to the particular design and location of the trigger spring, making it subject to recoil forces generated when the rear of the barrel impacts the locking block directly above it. It dampens the vibrations experienced by the trigger spring (called harmonics by some gun engineers) and extends the service life of the spring. The 3rd gen's have a different design and don't require a vibration dampener.
Absent pictures being posted, I could only speculate the previous owner of your 3913 didn't routinely and fully clean that part of the frame, so some dust, fibers and other debris may have had the opportunity to accumulate and form what you're trying to describe.
As an armorer, I've seen some 3rd gen's which appeared nicely clean on the outside (and even inside/outside the barrel), but which had accumulated some nasty collections of dirt, fuzz, hair, dust, congealed lube/solvents and other bits of unidentifiable debris inside the frame.
You ought to see the inside of the guns carried by K9 officers, as they seem even more prone to accumulate dog hair in the oddest places, and the congealed hair and dirt can start to take on a mysterious greenish hue after a while.
Now, there's been some sort of aftermarket "buffers" advertised for sale for S&W 3rd gen guns, at one time or another, and while I've never seen or used one, I'd not be surprised to learn of some owner thinking to try something of that nature in their gun.