This is a modified Original Prusa MK2 fan nozzle shroud.
The original fan nozzle did not fit well and also directed too much cooling toward the heated nozzle. This resulted in THERMAL RUNAWAY and lowering of the hot end temperature during printing.
I fixed my THERMA RUNAWAY ( and improved my PETG printing) problem by doing 3 things:
1. Using the newest firmware version 3.1.0 (Nov 2017) there is a thermal calibration for the PID. I did a calibration for 240 degrees (for my PETG filament) and a calibration of 215 degrees (for my PLA filament) - this calibration will help stabilize the temperature during printing
2. I changed the FILAMENT SETTINGS under the filament setting tab in Prusa Slic3r version 1.37.1 - for PETG the cooling is MAX 30% MIN 15% DISABLE FAN FOR FIRST 3 layers - for PLA cooling is MAX 80% MIN 75% DISABLE FAN FOR FIRST 3 layers
3. Replaced the fan nozzle with the version included here