It looks like a 13mm PS13 in which case I think you'd need rebuild kit no 10436250 (the new number for, what was, 10436270).
Eg.
https://addictionmotorsport.co.uk/br...-ktm-110436250
or same item on eBay
https://www.ebay.co.uk/itm/Brembo-13...-/115092093893
Or Ducati Paddy is still showing the old number 10436270
Also, this thread might be useful as it contains Mr Gazza's experiences of refurbishing his own master cylinder.
http://www.ukmonster.co.uk/monster/s...ad.php?t=57129