Every few months somebody posts one of these scooters on the interwebs and people are immediately confused and some even go so far as to call them bodges. The reality is, these are extremely rare machines and only sold in the US and a handful of other countries. The primary reason for this is the original square headlight found on Indian and Italian Lambrettas did not meet the updated DOT standards during the 1970's as the headlight could not be aimed left to right. SIL's solution to this issue was very similar to what Serveta did with the use of an SX style headset that they fitted with an adjustable headlight. The reason for this was the adjustable round headlight glass used in the Serveta (along with later model Vespa) was commonly available already so SIL just needed to cast a different headset top and bottom to fit these.
SIL may have borrowed the designs almost wholly from Serveta as these machines even include the large "space shuttle" CEV switches found on Servetas. The base of the headset casting is slightly different from what Serveta used as the bottom had lugs cast in for signal mounting.The back of these headsets are also slightly different from their Serveta counterpart.These machines also included a variaton of the CEV tractor light tail light as found on Servetas and had a special fuel tap lever as DOT regulation required the fuel tap lever to point forward when in the "On" position, which is why we see the use of the bridge mounted fuel tap on Servetas of the same era.
These were only supplied with 150cc engines which also had a curious neutral switch that SIL designed to meet another DOT requirement. Basically it's a rear brake switch mounted to special braket that operated by moving the cable butterfly:
image credit: Craig Bachler
US market SIL GP's were only imported for a brief time during the late 1970's-early 1980's. It is unknown how many were actually brought in during this period.
Edited by theageofindustry