Nicely done to find and identify it
The spitting horse was pretty popular and was selling around 10-25 dollars for mint state examples a year or two ago and still sell in circulated conditions. Regardless its a nice keeper also for a collection and the added things you found just adds to the coin. Lots of general errors have been creeping up in values this year so check ebay sold listings for current values.
Heres some general info on it I have:
On some strikes of the 1999 Delaware quarter, a raised line can be seen extending from the area of the horse's mouth down to the left. It can be seen between the letters "C" and "A" in the name "CAESAR." This line was caused by a prominent die break that made its appearance on a number of these 1999 Delaware quarters.
It's unknown exactly how many of these errors were produced. (There is no published mintage for an error coin.) They at one point were considered rare State Quarters
and, upon their discovery, were selling for as much as $250 to $500.
Over the years, the 1999 Spitting Horse Delaware quarter has proven more common than initially thought. Interest in this coin has also waned some, dragging prices downward. But this fascinating variety still fetches $10 to $20.