SA5941001 GT Series Rubber Cap
Details
OEM Brother GT Series Head Parking Rubber Cap with Drain Tube Extension. NOTE: We strongly advise replacing the "Cap Holder" at the same time.
This is PART #1 in the diagram: 
OEM Brother GT Series Head Parking Rubber Cap with Drain Tube Extension. NOTE: We strongly advise replacing the "Cap Holder" at the same time.
This is PART #1 in the diagram: 