A "1" in the Ω : 565 caliber question

Means the stem that the hands go on are larger.

So a dial with Onyx inlay on the markers Or bigger markers will usually have a 1 inside the Omega symbol on the movement

As noted this refers to what is known as the "hand height". In this case it means that three parts of the movement (the sweep seconds pinion, the cannon pinion, and the hour wheel) all come in different heights in order to accommodate different features on the dial, such as applied markers.

Many movements have this sort of variation. For example the very common Omega 1120 movement has 4 different heights available. This doesn't include other variations of the movement, such as the 1128 that has the GMT function added, that have their own variations.

Cheers, Al