Hi 30mhurco,
Your revolver was manufactured and shipped in December, 1864. Don Ware author of Remington Army and Navy Revolvers 1861-1888 identified Giles Porter as the principal sub-inspector using the GP cartouche. Some time ago I seem to remember reading where that cartouche may have belonged to George Palmer but I an unable to locate that document.
Hope this helps,
Billt