Question about Omega Speedmaster

[njlam]

I am new to the Omega brand but the Speedmaster Moonwatch Professional Co-Axial Master has caught my eye.

What does Co-Axial mean?

On the Omega website there are several models listed under this name

The one with model number 310.32.42.50.01.002 has CoAxial

and another that looks identical (311.33.42.30.01.002) does not.

How are these watches different?

Generally, are these models easy to get (available new from ADs), or does one have to go grey or preowned to pick one up?

Thanks in advance.