BAR is correct the size of the blade is on the top though you may want to get a couple of different sizes + & - as well due to the fact your shortening the sight radius from the original configuration therefore what worked on the longer barrel may not on the shortened length and the changes in the harmonics of the barrel that having the flashider plonked on it.
There are plenty of sight blades for sale on that site in the UKicon cannot vouch for the USAicon just ensure you get the ones for a No.4.