Factory images for the March 2017 security update for most supported Pixel and Nexus devices are now up on Google's developer sites here and here. That includes the Pixel, Pixel XL, Nexus 6P, Nexus 5X, Pixel C, Nexus 9 (+LTE), and Nexus 6. Builds for the Pixels are NOF27B for all variants except devices on Rogers, which should use build NOF27C.

The Nexus Player appears to be left out of the March updates at this point.

You can find the accompanying security bulletin here, which as usual lists a significant number of security threats that have now been eliminated.