[chris vickery]

Was there ever an artillery school or range in the area?
If not directly in the vicinity found it could be an overshoot out of bounds.
If in fact it is a 6", remember these guns were used almost 100 years ago and the history could be lost.
We have had a few recent "rediscoveries" in recent years of long forgotten about bombing ranges, usually when construction companies decide to develop areas for new housing projects.
I find it odd that a souvenir would end up buried in a field. Maybe found in a barn or shed it would make sense but ploughed up doesn't unless it was deliberately put there.