BTW...several sellers on Ebay advertise "white backs"...especially siriussportscardsauctions who has several...but all seem to be "cream" to me, or match what I have. Either they are confused, or maybe the "white" versions you've pictured or others have seen are just really clean cream backs?
Again, I've put several 1960 sets together and never heard of white backs so no clue here...
https://www.ebay.com/itm/18201873282...cAAOSwWMhagnff