Welcome SOF:
I think what you have there is a Spanish Enfield made in Madrid, Spain (Spanish Model 1860 Rifle). I cannot see the rear sight by your photos, but if it is clamped on the barrel, this is a sure sign it is Spanish. From what I can tell from your photos, it matches the Spanish Enfield seen in “Firearms From Europe,” second edition, by Whisker, et al., 2002. Pages 136-138 I think show your weapon with the three screws and a sling swivel mounted on the heel of the trigger-guard. “Dates of 1861, 1862 and 1864 have been observed.” Its American Civil War usage is uncertain: “William Edwards’ ‘Civil War Guns’ credits the model with Civil War usage but doesn’t cite any proof of its alleged use. Union ordnance records do not contain any listings of Spanish arms. It is also likely the arm was obtained through the Liége trade as it was manufactured in the Madrid arsenal . . . There is a possible Confederate association”--perhaps through Cuba.