This bar adapter allows Stihl bars for the MS660 and MS661 to be installed on Echo chainsaws. 
 
For reference: 
Echo: 8mm bar studs
Husqvarna: 9mm bar studs 
Stihl Ms660/661 and other large Stihls: 12mm bar studs
Stihl MS880/MS881: 14mm bar studs.