A foresight riser is not possible for a Model 75 as it has a one piece cast foresight assembly/housing that fits around the barrel, but the '75 HV came with a higher optional front casting as well as the standard height type. Both came with every new rifle.
Your only "good" option for "dual distance" shooting is a spare rear dioptre, but it does not have to be from a Model 76 as the other Original dioptre sights will fit and work just as well and there are a lot more of them about so you should have more chance of finding one.
I may even have one - I will take a look and let you know if I can find it.