I believe the iron object you found is indeed a broken-off section of an 18th or 19th-century bayonet's blade ...because it has the typical "wide triangle" profile of blades seen on most of the socket-type bayonets in that period of history.
I agree with SwampCreek that you also found some Pinfire-type 10-gauge shotgun cartridge bases. As you guessed, they do not date back to the American Revolutionary War era. However, they do date from the 1850s into roughly the 1870s. (The Pinfire primer-system rapidly fell out of favor after the advent of Centerfire-primer cartridge ammunition, which went into widespread production and availability in the 1870s.)