Immobilser, doubt it was factory fitted somehow back in 1984, even most TV/video remotes were still on a lead back then.
But then who knows. I did have an immobiliser on a 1976 Porsche I had a few years ago, was a big box slung under the dash with dial numbers on, so you dialled the code in pressed a couple of buttons, like on a briefcase and off you drove. Was called 'The Ungo Box'