This is as basic as the Seiko chronos got. The movements did not vary at all, the 6139A was superseded by the 6139B sometime in the early 70s with some minor technical changes under the dial but the only difference in the many models that Seiko produced was only in the cases. This one has no bezel, no tachymeter scale only a chapter ring that is part of the dial. It’s also one of the few Seikos of this era with a perspex rather than a crystal glass. So I presume it was the base model.