My 1955 D150 has a frame number of: *21706* i 150 D , (where the i is the Innocenti logo), positioned under the tank on the right side as you sit on the scooter.
This falls in the range of 611 to 33148 for a 1955 150D as stated by Vittorio Tessera in his book "Lambretta Restoration Guide" 2012.
His book (and the Pete Davies book "The Lambretta Bible") mentions only "Frame number prefix", whereas mine is a suffix. Does anyone else have the model as a suffix on their frame followed by the production number, or know anything about this please?