Hello,
From an older topic I know that the internal radii seems to vary between 3.6 and 3.8mm.  The outer radii varies between 4.8 and 5.2mm.  That has been measured by hand. Is this information still true today (any process improvement?). 
What about the tolerance of the 20 mm of the side of the tube ? Is it +/- 0.1 mm ? In my application the tube shall slide inside the groove of roller which looks like this one <https://www.aliexpress.com/item/10-56-31mm-belt-groove-H-type-injection-molding-machine-safety-door-roller-square-groove-D10/32816622292.html>. The groove profile shall fit the tube side profile as close as possible. 

See you talking about the pultruded box section tubes? If you are thinking about some sort of structural mechanism I would advice against using them in that manner with roller support. Unidirectional tubes are really easily damaged when it comes to force transmission points. For very low loads it might work well.