US Bicentennial State Flag Stamps

I have an entire sheet of all the 50 state flags. Seems to be in good condition, between 2 cardboard sheets,13 cent denomination.There's a twist to this one. Has the cancellation mark with the date July 4 1976, Topeka KS post office(My uncle used to be a mailman).

Does the cancellation mark- which is multiple times, each mark covers about 4 stamps- add any value?

I got these as a gift from my uncle way back when.

Short answer: No.

Longer answer. The state flag pane was released in February 1976, so your item is not a first day of issue, but rather a later July 4 souvenir. To give you some idea of the market value, there are several First Day panes available in the $10-15 range on ebay including shipping. Most of the demand for a non-first day item would be from a Topeka collector. If you are offered $10 for it, take it.

"between 2 cardboard sheets" is scary.
Unless one has "archive quality" pristine white cardstock for backing, eventually the material in average paper esp. cardboard and anything "kraft" or manila or darker will migrate and "tone" the paper collectible trying to be saved. Sorta same color as tar from tobacco smoke staining walls etc.
(You can check with heavier paper stock and the like at OfficeMax, Office Depot and the like for brands that claim "archival quality.")