As ALL (all!) manufacturers did during the 1970s, Piaget also reduced gold used in some watches due to its price-explosion (USD35 in 1970 --> >USD800 in 1980) and so the later in the 70s or even in the early 80s the thinner the Emperadors -- the difference is hidden behind a same frontal-appearance but the back and side reveal it: #SameOrDifferent? -- nb: the winding crown of ref9131 in the left part of the photo is simply hidden on the other side -- this is not a difference. But the difference is very obvious when you see both next to each other. Your friendly dealer might not point you to it, he might even try to hide the photos from the back and side to give you the front-appearance of the real thing, and esp when he has a thin version for you; but make sure you have at least seen the two variants before you make a decision. It is not a difference like day and night; but like summer and late-autumn.

Interestingly the 1979-arriving Piaget Polo went one step further than even the thin (left) Emperador: less weight, less quality, simplified finishing;

By the way, there is a very funny Emperador-look-a-like by Patek Phillipe that mimicks the successful Piaget Emperador but instead of being heavy and solid (and costly in manufacturing) it is all hollow and the links are threaded on two gold wires... do you know its reference?