So maybe you bought an early MGB positive earth electric tach and found that it doesn't have the green wire grounding connection inside? Then you may have one of the more rare positive earth units with different inner parts, used in late 1967 MGB production.

RPM X100 4 CYL POSITIVE  EARTH RV12401/00B SMITHS MADE IN UK

Note terminal style, screw locations, and the offset illumination socket at top.

For instructions to convert this type of tach from positive to negative earth, see here: http://chicagolandmgclub.com/driveline00/0505/mgb_tach.html