Speculation not knowledge. I have heard that the Skillet racks were given out for buying a set of skillets. I would guess that skillets were sold in a variety of packages. and the cheapest were in sets like 3, 5, 6,7,8, and 9 plus a rack or in bulk. A box of 4 #8 for example. So I would guess that most skillets were bought by retailers in either sets of in bulk. And if someone specificly ordered a pan with a cover it may be shipped in it's own dedicated box. Or the retailer may have just ordered a set and broke it up for the purchase. Due to this reasoning a Chrome or Nickled skillet whould be more lilkely to have it's own special box. As the plain skillets were a little cheaper, and more lilkely to be ordered in sets or bulk. A retailer is not going to want to order a set of Chrome skillets which they will have to sell the extras at a premium over plain skillets. Just like they would not want to have 3 extra chrome skillets to sell at a premium. Of course some retailers may have gone thru many crome skillets in a months so they would buy in bulk also. This is not to speculat that no chrome or nickled skillet were bought in bulk. But I would guess it would be more likely to hapen that way.
This would also help explain why the # 4 is a rarer skillet. The most common set sold did not include the #4. This is all speculation. There are probibly smarter people that will chime in who actualy have boxes. I am looking farward to hearing what they have to say.