A correction or refund in the 37th to 48th month gives CBSA five years: do you still file it?

Strictly speaking, paragraph 17 does not only bite on refunds. It names corrections in the same breath, and a reason to believe can arrive in that last year as easily as at the start. Only the refund is timeable, which is the real half of your point.

An obligation you owe and a right you may exercise are different animals, and the Customs Act itself relieves you of the correction duty where the correction would produce a refund claim. So the extension reads less as a penalty on the right than as time bought on a transaction you have just reopened. Whether that was designed or fell out of the drafting I cannot say. I teach agreements, not Canadian administration.

Your axis is right. The lever is when your desk looks, not whether you file.

same call our side with disclosures. never the months, always what else was on those entries. clean file, the extra year finds nothing, take the money. dirty file, you were fixing them all anyway.