I have a very early Colt SAA with the "1" below the SN's. It has original ivory grips. Ron Graham surmised the "1" was used instead of an "I" by the colt person who did the stamping. Maybe at some time in your gun's 'life' it had ivory grips.
I believe it to be an inspector's mark. Curious that both bati's pistol and mine have no mark on the barrel lug. In my case I thought that it was because the pistol has no barrel address and thus did not pass inspection on that part. As these were being turned out when development of the SAA was in full swing I figured that it must have been thought unimportant to send it back to be re-done so they just shipped it as is. What are your thoughts?