Why 650s changed to two truss rods?

[Yuanjia]

I always wondered why the early 90s model 650 I have has one truss rod and every other one I see has two.

Did they change them due to neck stability? I’ve never had to adjust the truss rod on mine which is amazing because all of my far east made guitars need an adjustment every season.

I imagine the double truss rod versions have thicker necks and would accommodate thicker strings, so I might be looking at a black one for baritone tuning if the neck isn’t too thick.

Anyone have any knowledge or anecdotes about 650s over the years.