https://aviationweek.com/trine-develops-hushkit-keep-older-learjets-flying
Trine Develops Hushkit To Keep Older Learjets Flying | Aviation Week
to keeptrinehushkitolderlearjets
https://www.ainonline.com/aviation-news/business-aviation/2015-12-29/learjet-20-xr-mods-now-have-hushkit
Learjet 20 XR Mods Now Have Hushkit | Aviation International News
Deadline for Stage 3 noise compliance was Dec. 31, 2015, and now all Learjet 20 series can be modified to meet the requirement.
learjet20xrmodshushkit