You either had gauges or idiot lights in '79, and gauges were a very common option.
Take a look at the gauge pictures on the truck web site to see the gauge design(s) for various years. By that time I believe the entire was all electric, and had a voltmeter.
In my '75 truck I have the older style cluster, with a mechanical oil pressure gauge and an ammeter. That is why you can't always swap clusters between years.
Bruce