Thanks, Ron. For the purposes of the inventory, it makes perfect sense to go with the most likely time period — when ambrotypes were most popular, and that will basically narrow it down to a decade (circa 1854-1864). Beyond that — when there’s time — we can pursue research on when and where the photographer worked, as well as see if there are any records that show when the fence was installed or removed.
And about opening the frame — that was my fear and inclination as well. I think I’ll leave it be.