With Glenn’s help from the Rokonworld Message Board and the www.smokstak.com forum we have been able to identify where the MK0 muffler came from. The part number on it is IGM. 0277 S and here is the Innocenti factory drawing. This muffler fits 1958 through 1962 Lambretta scooters. The exhaust outlet was moved to point to the rear and the original outlet has a patch welded over it. Obviously the Innocenti seat came from the same scooter.
This also appears to be the same muffler that was used on the 4a Maico bike.