Looks like Glock is offering to swap slides for European customers wanting to go to a MOS configuration.
MOS slide
Upgrade for mounting of popular optical sights
The MOS replacement slide is offered for several models* to upgrade your standard slide to a MOS slide. By choosing this option your slide will be exchanged and your weapon serial number stays the same.
Interesting option; would prefer to see them offer MOS slides for sale like SIG does with the 320. But a step in the right direction; wonder if we'll see this offered in the US.