The FT035 Grounding Spring is situated above the FT027 O-ring, highlighted by the yellow arrow below. It contacts the outer edge of the mounting pipe (or size adaptor) directly after the Tapered Pipe Insert (assuming the mounting pipe is conductive and not coated):
To facilitate a good connection please ensure any surface contamination or oxidation is removed from the mounting pipe prior to installation (this is easily achieved using Scotch Brite pads).
The FT035 is typically fitted at the factory when ordered by customers, but can be carefully removed or added using non-marring tweezers by customers wishing to modify the grounding scheme of the sensor.
Refer to the user manual and the following Application Notes for more information:
A4166-1 FT742-DM Accessories
A4167-1 Lightning Guidance