The error is due to over-inking of the printing plates. 1976 $2 notes don't usually support a premium, and I don't know if the error will give them any premium. Being sequentially numbered may give them a premium, too. Check eBay for possible values. Look at completed sales, not what sellers are asking.
Thanks for your help! My grandparents collected a bunch of paper and coins...going through all of it trying to learn the significance of what they collected.