Is there any chance of posting a good clear picture of that butt marking?
It sounds like the butt was "cleaned" at some point and the markings that were less deeply stamped were removed. But at least a picture would help determine if there could be anything there and if the spacing is correct.
There would not be two lines of serial number, just one, as the guns inspected by Kimball never went past four digits. Further, the number was not as deeply stamped as the Colt serial numbers on later guns. When I said
"line", I meant just that, two crosswise lines bracketing the serial number.
I would send you a picture, but it would require violating copyright since it would be out of a book. I have several of the New Army and Navy revolvers, military and commercial, but no Model 1899.