Presumably this is a '78 T140E model (with the vertically-split R/H switchgear)?
From your description, the item appears to be the front brake light switch, which would normally locate inside the cavity of the master cylinder housing and is then retained by one very small screw.
The switch 'prong' projects through a hole in the master cylinder housing adjacent to the brake lever where it contacts the tip of an adjuster screw inside the pressed steel lever.
If you re-fit the switch, take all necessary precautions to ensure that the soldered wire connections on the switch cannot touch the master cylinder housing if the switch should move out of position slightly, as a dead short and constantly blowing fuses will be the result. So I suggest you put some kind of electrical insulator between the switch connections and the alloy housing to prevent that from happening.
