All of the cartouche associated with Remington firearms during the civil War were either 2 or 3 characters within a rectangular or oval border.
There is a two digit cartouche in your serial number range,
GP Giles Porter. APrincipal Sub-Inspector. Porter seems to have served at Remington on more than one tour of duty during the War, as the serial number range of the revolvers bearing his cartouche are spread out from 20,000 to over 100,000.
Serial number 108931 shows a November 1864 shipment date.