Loop collars from my knowledge were normally used on plain collar shirts, the most familiar being something like a Pendleton Boardshirt, a typical Hawaiian shirt, or something like the one below.
This example though features the normally more sought after button-down collar, but with the added edition of an off–centre button under the collar to secure the loop. I’ve never seen this type of hybrid before, and thought it was worth documenting. I quite also like the novelty print of the fabric. The shirt is available to buy at this Etsy Store. (I’m not the seller of this shirt)