This item is the LUCAS 6RA Relay (SRB111).
 
This relay features a normally open, single-pole, single-throw contactor configuration with a maximum current rating of 20A at 12V.
 
The connections are located underneath, in a space terminal form factor, with two mounting holes situated at each horizontal side of the casing.
 
Pictured is a Mark II Jaguar with this item installed to replace the horn relay under the bonnet.