You will get more meaningful responses if you give more detail of what you are looking for. These are collected mostly for novelty reasons and dont have much value unless you have a scarcer post office and the hardcore collector who wants it. These are bicentennial postmarks from July 4 1976 from Buffalo NY. April 13 1976 is the morecommon date. The extra stamp probably does not add much. It might have a little more value if one postmark was April and the other July.
I was ten years old when me and my dad went to the post office to get them stamped and postmarked, I was looking for a value they're not leaving my possession, that was 41 years ago.
I would think postmarked on the 4th of July would be a little more meaningful than any other month considering it's a bicentennial year.