As far as I could investigate, the earlier mention of the Exercise Wheel device in promo advertisement was in Popular Science, oct. 1969, Vol. 195 Nº 4 pg 96. I assume it has developed itself naturally from the anti-extension exercises already done in gymnastic balls, barbells and dumbbells (both already with rotational shafts) .
The advantage of centring the wheel allows the manufacturing of a lighter and cheaper device compared with a dumbbell. However, when doing the one-handed variation of this exercise, you may need a double wheeled exercise wheel, just like the dumbbell it came from.
Popular Science out 1969 Vol. 195 N 4 pg 96
More info in: http://www.beastskills.com/ab-wheel-rollout/
Website de Pedro Figueira 