You are probably being offered one of the "battlefield pickup" or "parts" Nepalese models currently on the market. There are two 19th century Nepalese models, the Gehendra and the Francotte. There are also Afghan "Khyber Pass" copies which are still being made today and sold to unsuspecting Allied troops. All of the parts are hand made; they are usually not interchangeable. For more info you should visit the dedicated forums
here and
here.
Here is my Nepalese Francotte Martini-Henry: