Why is there no article before bouts in this sentence?
Un peu plus tard passèrent les jeunes gens du faubourg, cheveux laqués et cravate rouge, le veston très cintré, avec une pochette brodée et des souliers à bouts carrés.
Motivation for the question
My French grammar book says, "It is so usual to put in the article, that perhaps it would have been easier to learn when to leave it out," and goes on to list the exceptions, e.g. in proverbs, in lists, in titles, often after de etc.
But none of them seem to fit bouts above.
My one (wild) guess is that it might be the influence of set phrases like à bout de souffle and à bout portant.
In sum, I would like an answer to explain which recognized exception applies to the above case or, if none apply, what considerations make it "sound right" without an article. Thanks.