I have owned several Model 75 over the years and have always removed the standard "adjustable" type of front dioptre that relies on squashing an "O" ring to give a limited range of apertures.
If you unscrew the foresight assembly and remove all of the "innards" you will find a small disc with a pair of springs attached - one on each side.
Get yourself a front sight insert of the correct outside diameter to fit inside the foresight tunnel and fit it into the tunnel and then put the spring loaded disc back in to retain it and screw it back together again minus the red plastic assembly that includes the "O" ring.
From then onwards it is easy to change inserts to find the size you like to work with.
Rossendale Target Shooting Club. Every Tuesday and Thursday evening 7 - 10pm.