I recently acquired a copy of Rogers Postal Booklet Catalog, published in 1947. For what Scott calls BKC2 (37 cents, 2 panes of 3 6 cent stamps), Rogers distinguishes two varieties, one with "Thick" covers, and the other with "Thin" covers.
The valuation is listed slightly higher for the thick covers rather than the thin, so I would presume from this that the thin covers are more common.
I have three BKC2 booklets, and I cannot see a difference in thickness, not even with calipers.
I'm not on a search here for something worth a lot of money. My interest is more abstract. I am trying to complete a collection of as many airmail booklets as I can (given budget constraints). It would be cool, I think, to have representative examples of both "thin" and "thick" in the collection. But I've never seen dealers online distinguish the BKC2 booklets in this fashion.
Has anyone ever seen the BKC2 booklets distinguished this way?
I am beginning to wonder if the Rogers catalog is simply wrong about this. It seems to me that if this were true, there would be some other evidence of it. I am hardly an expert either, with no experience to go on other than the ones I have, and they are all the same.