VMI88,
The only Navy Arms marked Mississippi (and "Zouave") rifles that I have encountered were made in Italy by Antonio Zoli & Co. I do not know if they used any other makers for these models but can't rule it out. Are there any markings visible at all at the rear of the barrel?
Usually I see "Navy Arms Co, Ridgefield, NJ" and the Zoli logo that looks like a bow and arrow The Italians also used a Roman Numeral code and later a two letter code to indicate the year of manufacture. For example XXV is 1969 and AE is 1979. These may not be visible without removing the barrel from the stock. If you Google "Modern Black Powder Proofmarks, Unraveling The Mystery of Icons" there is a good article by Dennis Adler with lot's of good information that will help with your question. If you have time to post any photos we'll be sure to check them out.
Bill