It is my guess that all 68's came w/ arm rests.
Actually there were some cars that have what you may call a limited production run.
Some cars may have the screwed on arm rests. Similar to the front door arm rests. These are what 68's should
have came with. Then there are some cars that have a rear arm rest that is a wide panel that attaches over the rear panel that basically goes down to the floor and has a metal spacer type panel that goes up to the top of the rear seat back. That is what my car has. It came w/ a factory installed Buick interior. That is what I would call the limited production run. Mostly April 68 built cars.
Since the wide panel arm rest goes down to the floor the rear seat is actually narrower then the regular
Chevelles. If your cuz is having troubles with seat covers have him check the trim tag to verify he doesn't have a Buick or even an Olds (narrower then reg Chevelles) seats. The trim tag will have a seat code then a dash then another seat code.
At this time this is the best pic I have. http://www.chevelles.com/showroom/mild68ss/Buickinteriorrear.jpg