MB designed Viton Lambretta carburettor mounting rubber, a simple way to swap carbs if you want to go smaller from say a 28/30mm Dellorto down to a 25/22mm Dellorto, Or you can go up from a 22/25mm Dellorto to a 28/30mm. 
 
No need to remove and replace the inlet manifold. 
 
Large end diameter of spigot is 36mm, with one groove
 
Small end diameter of spigot is 30mm, with one groove
 
The rubber measures 32mm in length.
 
 
These may come in black or brown but will always be Viton