Correct, orginally there where no dimples. There were however a small lot of 7.62x39 rifles brought in a few years ago that did indeed have dimpled receivers. Those were explained as using over-run mil-spec receivers from a recent contract. That theory is supported by the Y (left side) and I (right side) stamps were the auto sear hole would have been drilled. Normally Saigas do not have this, even the newer ones with dimples lack that feature.
Dimples were added to newer Saigas mostly due to request from US customers. Maybe it does reduce cost as the factory can use the same line for either receiver now, minus a few machining ops, OR more likely they learned idea of demand = more $$$ both in sales and increased price with miniumal increase in cost.