'70s Unopened Rack Pack Question

[Kurri17]

As I have gotten more into unopened, specifically picking up the '70s stuff, I have noticed and wondered about the plastic wrapping of the '70s rack packs.

Please note I am not questioning the legitimacy of any of the packs I am showing as examples, only using them as examples of the different plastic for purposes of illustrating my question.

For example:
http://www.ebay.com/itm/1974-TOPPS-B...wAAOSwBj5Zxnvv

This pack is as I remember from the untold number I bought as a kid. The plastic is a thicker plastic, and was a pain to open, and you can see some aging which seems normal to me. Note the 'shrinking', for lack of a better term at the top and bottom of each card section, sort of an inward scallop shape of the plastic.

This one:
http://www.ebay.com/itm/1977-TOPPS-B...MAAOSwAVxZxn6z

Obviously this is from three years later, but you can see the difference in the plastic wrap. I have noticed the same thing with football rack packs as well.

Does anyone have any knowledge regarding the difference in plastic used in the rack packs? Did they change their grade/type of plastic wrap used at a certain point and if so, when?

Any input would be appreciated? Thanks.

Rob