Mansfield Flush Valve Stop Cap 225-5907 Colors vary but they are all now white. This is the part that screws on top of the flush valve and keeps the flush tower from rising too high. When it is broken the flush valve can get hung on it preventing the tower from falling back down to close the valve. Many times when this part is broken it happens because the toilet is hard to flush and replacing the flush valve seal will make it easier to flush.
This product is available.