Only if this is literally copied from a (non-)working script: the property is called 'columnsPositions' ...
You have eagle eye!
Wahay! Jongware scores again! Thanks!
BTW, Is there any way of getting different-sized columns without scripting?
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Yes. Deselect "Lock Column Guides" (under View).
Ah yes -- now I see it!
FWIW, in case any amateur like me finds this thread when trying to solve a similar problem (I'm sure it's already obvious to the hotshots!)
"columnsPositions" expects pairs of column guides, left and right, so the following wouldn't work (even though, thanks to Jongware, it's spelled correctly):
myDocument.masterSpreads.pages.marginPreferences.columnsPositions = [100, 200, 300];
But (I think) this would work:
myDocument.masterSpreads.pages.marginPreferences.columnsPositions = [0, 99, 101, 199, 201, 300];
That should make three columns with a 2-unit gutter between them.
No idea why that happened. Something or other strikes again.