Ducati MULTISTRADA 1200S ABS 2015 Workshop Manual page 826

Refitting the secondary air system
Note
Remove cloth from the secondary air system duct on head cover.
Warning
Do not invert the positions of the covers and components of the two valves of the secondary air system. Refer to
the reference notches present on the secondary air system valve cover and on head cover.
If disassembled, refit secondary air system valves (10) applying the same procedure for both of them.
Fit the spark arrestor (14).
Fit the reed valve (13).