Hi stonfre.....welcome to TFF!
Well, looks like you've got a rather special firearm here.....literally!
You have what's known as a Colt Model 1861 Special Musket, also known as the Special Model 1861 Contract Rifle-Musket. These were made from 1861 to 1865 and about 152,000 were made....of which Colt made about 75 to 100,000.
Yours was made in 1864, as indicated by the date on the lock.
Here's some interesting info on your gun:
According to Flayderman's Guide to Antique American Firearms, yours is wortrh:
Very Good - $1,400
Excellent - $3,500