The standard swaybar dia for the front of a 1200 is 17mm on all models as far as I'm aware. The B210/120Y has an 18mm bar is is a popular swap. Mareo you might have a custom bar or a factory race item. My GX has a Datsun Competition front bar of 20mm dia and a "factory option " 12mm dia rear bar. I think that in days of old when shocks and tyres weren't so good the rear sway bar was one way of taming the the handling of the 1200s on the limit but on the road I can't see any real advantage. We've come a long way in 30 years but the basis chassis of the 1200 is still close to perfect...