From what I remember Murray ( Mountfield) used a couple of different setups for securing the blade but I think that the part number for the nut was 15×72. Chances are that is an old number but may be a starting point. It or it’s replacement may still be current but if you draw a blank try Jon Cruse at the Hailsham Mower Centre; he specialises in old stock for all makes.
Edit. The parts book shows it as 1/2” x 20tpi which is a standard UNF nut size.