What's the relationship between Autopilot/EKF2 and ECL/EKF?

The EKF directory seems to have been much more recently updated in the PX-4 Autopilot repository whereas the ECL-EKF version seems to have been last updated several years ago. PX-4 documents are referencing ECL-EKF, but I wonder what the core differences are. I am assuming it was forked at some point but started diverging eventually? What’s the history behind this?