NGK BR10ES 'long reach' spark plug ('RESISTANCE' TYPE for use with SIP speedo's) 

The grade listings and applications for NGK spark plugs are as follows : 
B7*S -  town and city use / for a standard engine 
B8*S -  local and distance use / for a standard or midly tuned engine 
B9*S -  competition or distance travelling use / for a tuned engine 
B10*S - competition or full race use / for a high tuned engine / sidecar use 

Spark plugs with a 'BR' denomination have a built-in resistance and are designed for use with delicate electronic equipment (such as SIP rev counter / speedo units). 

Spark plugs with a 'HS' denomination are SHORT reach versions (usually utilised for early pre-S1 Lambretta models manufactured between 1947-'57).

Lambretta models that use a SHORT REACH spark plug are: A + B + C / LC + D / LD + E /F + Lambrettino 48

Lambretta models that use a LONG REACH spark plug are: S1 + S2 + S3 + GP / DL + J (all versions) + Lui Vega Cometa + Serveta