Fender has done various little things with the tele over the years, so it could be a factory job, it could be a mod done by its owner. I don't know of any model like you describe from the early 80s, but there's lots I don't know.
One thing you might try is to remove your control plate and then remove that toggle switch. If it's a factory control plate, they would have drilled the hole and then chomed the plate, so it will look factory-original. The walls of the hole will likely be pretty smooth and without tool marks. If someone did the mod at home, there will be no chrome in there and it may be obvious that it was drilled from tool marks, jagged edges, chipped chrome, etc.
You can also remove the pickguard and look at the pickup cavity. I don't think Fender was routing for humbuckers as standard back then, so the route will have been enlarged and probably not finished. If it was finished, then it will probably look a little different than the finish on the rest of the body.