Hi Derek
I've finally fought my way through to the ME90 ( currently attempting to improve access to my workshop in the hope of doing something useful ).
The headstock indexer, as shown in Michael's post, has four components ( plus a few screws ). These are the index wheel, a machined rectangular mounting block for the detent arm , the detent arm itself which is a cranked piece of steel bar, and the detent pin which is a piece of turned and threaded rod knurled at one end and pointed at the other.
The pin itself is either chromed or more likely stainless steel as it is bright and shiny and shows no sign of corrosion, the mounting block probably steel ( based on appearance and colour ), the arm I am not certain about – possibly spring or hardened steel.
If you let me know what part you are missing I will see if I can produce some accurate measurements and also check any relevant screw lengths and threadforms.
Pero