Hello Michael,
The 5/8" dia torsion bars are the heavier type fitted to the 'Plus' machines (and available as an option on the Mark series).
As far as I am aware, there is no physical difference between the LH and RH torsion bars. Evidently, the thinking was that on initial fitting, the bars would take a 'set' as they bedded in, so should always be fitted to that same side (hence the colour coding). Thinking that through logically - assuming the bars don't get overloaded, they should only take that 'set' on initial loading, then remain the same. If you dont know the handing of your torsion bars, I would suggest that it would be OK to mark them up as LH or RH, and fit them as such. Then, after about 250 miles, remove a pivot bolt from one of the rear links to see if both links are still aligned and both torsion bars are taking an equal load, then adjust as necessary.
Hope this helps,
Regards,
Eddie.