I may very well be wrong, but I have never seen a '78 without the rear bar, including the 4 that have passed through my hands (302 Cobra II, 302 hatch, 2.8 coupe, 2.3 coupe), as well as a dozen or so base cars in salvage yards over the years.
Additionally, there is this from the April 1978 Ford Times magazine: "Special Value Edition Mustang IIs enjoy, of course, the many items that are standard on all Mustang IIs, like electronic ignition, front disc brakes, rack-and-pinion steering, rear stabilizer bar, tachometer and fuel/ammeter/temperature gauges."
I also thought that the '78 brochure made mention of it being standard, but I'll have to dig it out tomorrow.
As always, my position is that lots of strange things can/have happened to many of these cars in their 25-30 year lives.