Hey Ed,
I noticed that there aren't many proof marks in that chart that go earlier than the 1850's, and we know they were used well before those dates. I'm pretty sure a similar (if not exact) stamp was used WELL before the 1868 date stated on the chart. The stamp "E C P" under a crown is on a high quality over/under flintlock pistol I have that was probably made in the 1820's or 30's by Hole. I don't know what the proof mark is nearest the breech, but it's the same one on my halfstock fowler (that I'm also fairly sure is English, signed J.Clark on the lock, and London on the barrel). Please post some more photos, European guns are just as fun to look at as American (and often cheaper!).
-Eric