I can't see the pic but my guess is it's a meta immobiliser,3 prongs on a little MCB about 1/2 the size of your thumb?
Very common in the 90's and required by a lot of companies due to thatcham approval.
Usually cuts the starter and ignition circuits up under the steering column.
I got quoted £45 to have it removed on my golf but with a bit of patience it's easy to find the joins and solder/heat shrink them back to the original loom.