321 Galvanic Dial

Here is what Fratello says:

"Galvanized dial showing more detail
Looking at the dial of the Speedmaster Calibre 321, you will notice that it looks different from the Speedmaster Professional Master Chronometer Moonwatch. It has not been varnished like normal dials, but rather, it has been galvanized. This adds only a little more thickness but allows more detail in the dial. The circular graining of the sub-dials, for example, is more present in the Calibre 321 than in the varnished Moonwatch dials. We have received some questions about the Calibre 321 dial, and people often mention the “step”. Some wonder if the step is also possible because of the galvanized finishing technique. The Speedmaster Speedy Tuesday Ultraman, for example, also had a step dial but Omega used aluminum to make it that way. The current Speedmaster Professional has a step dial too, but this is because Omega has improved its varnishing technique."

https://www.fratellowatches.com/what-makes-the-omega-speedmaster-calibre-321-so-special/