Was the touch hole clear?
Given the dates of service, maybe it was an over 200 year old load, or was it used for other things before being put in the park.
An opportunity to find out the formulation of 1780's powder, if they would.
Now I REALLY want to clean out the Denver City park stuff (full of dirt, I did my handful when I was a lad). One 11" bronze Dahlgren, one 6.5" Parrot, and a Mortar. Put in place in 1895 or so by the GAR. I think the 11 is aimed close enough that when they put my ashes in a shell, I can come down on the capitol. Not to mention the 2 Napoleons in front of the Capitol, aimed at the City & County courts &tc. They have been worked on, so it is pretty sure they are clear.