Yes thanks Bob, I was quite amazed to see that lying quietly in the "long shed" near the Rotunda when I was there in 1987. I got a few photos of it, not great, but enough to suspect it is a 9-pounder field gun. If you can get any better pix of the marks I'd appreciate it. The thing that confused me was that I had always assumed it to be a War of 1812 capture piece, however the marks seem to indicate it was made by John Mason, who did not take over the Foxall foundry in Washington DC until 1816, meaning the piece would be 1816 vintage or later. The notes on it in some of the older British museum catalogs only say something about "presumed to be American."
