Aluminium upper plug 24mm; named assembly Art. No. 17.
This is the base upper cap which integrates a Schrader valve and an O-Ring (4).
This 24mm version is used in XC600, XC700 and XCR forks. Also used in all branded 26mm forks like Fuchs Sport, Kästle HDB01 a.o. Caps are in stock, if you need a replacement in time.
This is my own reproduction of this part in excellent quality.