Why do these parts not boolean union

Problem63.3dm (1.4 MB)
This has been giving me a devil of a time for a while. Why will these to parts not add together using BooleanUnion?

As always, if a Boolean fails, run Intersect on the objects to see where the problem lies - nearly always the curves from Intersect will give you a clue. The curves can sometimes be used to split and trim if Booleans do not work. In this case the intersection shows the coincident or near coincident faces are probably the reason.



This was puzzling because the frames here exactly match.