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 XC500, XC400 a.o. forks. Caps are in stock, if you need a replacement.
This is my own reproduction of this part in excellent quality.
When mounting the seals put crease between the inner lips of the seals before pushing the stanchions in!
“Some forks may require 5-10 hours of use before the fork fully breaks-in. After the bushings, seals, and other parts have matched to each other, the fork will move smoother. The tight tolerances make the fork last longer and work better.” (original text from Marzocchi Bomber Technical Information (2000) )