Hello Frank,
I have never seen or read about a US mark on a New Model Army Revolver but I have only been collecting for twenty years and have not seen everything. My guess would be someone tried to enhance the value of the revolver. As far as the serial number on the cylinder goes if you are asking if there should be one the answer is no. Remington marked the serial number in three places, on the bottom of the barrel, on the left grip frame and on the trigger guard tab. Donald Ware author of Remington Army and Navy Revolvers 1861 to 1888 believed serial numbers or partial serial numbers on other parts were used to match those parts to the revolver when hand fitting was required to make the revolver operate properly. I have seen cylinders with full and partial serial numbers on the nipple end. I have also seen other numbers on cylinders that did not match any part of the serial number.
Hope this helps,
Bill