The Prussian 1809 U/M (=umgeaendertes Modell = altered model from flint to percussion) was produced in the Prussian factorys Saarn, Potsdam, Neisse, Danzig and (privately)Suhl. Some were later rifled beginning in 1855 (as well as the M 1839) to M 1809/55 U/M and got an adjustable rear sight, because Prussia feared to be involved in the Crimean War before they got enough of the new needle gun.
Great numbers of 1809 U/M were imported by the North because Prussia indroduced consecutevly the needle gun, mostly over the Liège arms trade from Prussian arsenals. There is also a source indicating a small number bought by the Conferderacy (I always felt, that they didn´t purchase them, only picked them up on the battlefields).
As Greg told "OHIO" and "CITY OF PHILAD." marked specimens are known. Question to Greg: are there some pictures available of the US marked ones (flaming bomb etc.)? Never saw one; seems strange to me that the guys marked import guns at all and especially at the bottom of the barrel.
Best,
Marc