Then you were lucky!

There's no track missing from the dial - some 1st gen Carreras carried supplementary scales (Tachy, Deci or Pulso) whereas others, like this one, didn't. So it's a 7753 NS rather than an NST. The white ring you're seeing is the inner 1/5 second bezel with all the markings lost.

You do sometimes see the rings for sale, though they're getting rarer. And more expensive. With that and a service you will still have bought very well though!