Top quality, made by Koso.  This sensor / sender plugs into the wiring supplied with the speedo.  The ring sits under the spark plug on your Lambretta or Vespa - you will need to remove the existing spacer ring on the spark plug for this to work correctly.  Alternatively, you can place on top of one of the cylinder head nuts - this will read a lower temperature but it is less fiddly, or tap a hole into your cylinder head to screw in the sender (or temperature detector a SIP call them) directly.
 
Please note: do not force the ring onto the spark plug, you may be better fixing this directly to the head.
 
Five replacement rings are also included, but it' the sensor itself that is quite easy to break.
 
Product code: LUSPKI001R15