Dave, Richard,
I have considerable doubts that the gun is Prussian and a Springfield. St. Blasien marked muskets were made in the former monastry of St. Blasien in the Southern Black Forrest. It was - after the secularization under Napoleon - the gun factory of the Grand Duchy of the State of Badenia, Germany. They produced the military arms for Badenia, often used lots of parts of older French flintlock muskets (lock plates and barrels often bear French markings like "St.Etienne" or Mle 1816). There is - curiously - also a monastry named St.Blasien at Zella-Mehlis (former two cities: Zella and Mehlis) which never was used as a gun factory and often misidentified with the Badenian monastry. Guns produced in the Suhl region (not only for Prussia!) bear markings like "SUHL", "ZELLA" or "MEHLIS", never St. Blasien. Daves gun is presumably a Badenian rifled musket M 1813/40/52 UM, produced in 1851 as smoothbore, after 1852 rifled (and often restocked!). The "Springfield 1861" resemblance seems doubtful to me - it should have a larger calibre of .69 and French style bands, lockplate (some have flintlock style plates, others "modernized" with rounden edge) etc. When original it has an unique nipple: round with two slots for a normal screwdriver - but not few I observed have a Springfield nipple because the thread is the same. Many of them were imported during the CW -but mostly erroneously (also in the book "Firearms from Europe") identified as Wuerttemberg or French. Look at the barrel: a small "Z(shield)D" stands for Zeughausdirektion Karlruhe.
Best,
Marc