Different Klingon Foreheads

What’s the deal with Klingon foreheads being different?: Why do Klingons look different during Captain Kirk’s day than all the times after and before? What happened to their forehead ridges during that period? (We all know the real reason – the original show didn’t have the budget for prosthetic makeup … and starting with the movies they did. But that explanation is no fun!) … The 15th episode of Season