Brake Reservoir Switch. There is a PLASTIC tube instead of an Aluminum tube.
Fits:
420 Saloon
Daimler Sovereign
E-Type S1 3.8 – Roadster / FHC
E-Type S1 4.2 - Roadster / FHC / 2+2
E-Type S2 – Roadster / FHC / 2+2
E-Type S3 V12 – Roadster / 2+2
Mk X / Mk 10 - 3.8 / 4.2