As unsual Mr. Para is right about the 13. Here are pictures. 12c is in the diamond. Based on this I'd estimate the gun were bought in 1884. Sometime after 1887 the rifle barrels was rechambered and the shotgun barrel honed out. The shotgun barrel measures .724 at the muzzle. But it it were honed to .729, that would mean there is .005 of choke.
![[Linked Image from i.imgur.com]](https://i.imgur.com/Y6T7WcX.jpg)
![[Linked Image from i.imgur.com]](https://i.imgur.com/C2j4VhM.jpg)
![[Linked Image from i.imgur.com]](https://i.imgur.com/OP8d4z3.jpg)
Here is the Canadian owne'rs story of the gun and some more details.
The 18 is actually a 13 and the square appears to be 12 over 6 or c ? . As for the s/n on the sg it looks like it was upside down and then re-stamped [these pics show better] . The bore size at muzzle is .724 in [cyl bore ?]. Also originally there were addresses on the barrels , but i do not recall which one. Now for the rest of the story.
I acquired this old gun at least 10 years ago from a very elderly lady whose husband had been gone many years. He was originally of English origin and this gun was his parents/grandparents . The story she told me was that his relatives had left England and had purchased this Cape gun to go and live in South Africa . After some time they moved to Canada . The gun was mostly discarded and lived in an outdoor shed for over 40 years and was eventually stored in a closet in the house that she lived when i got it. The condition was terrible. The stock was completely broken off behind the trigger guard and the action and hammers were rusted almost solid. It took me months of soaking and building screwdrivers to fit he screws.The barrels and parts were soaked in a liquid i got from a gunsmith here which removed a great deal of the surface rust basically leaving it black . Before this the only indicator of the Damascus was under the forearm. I was then able to locate a fellow who uses a method similar to the one featured in the Double gun Journal to restore the finish on the barrels. This is when the addresses were lost as well as some loss of material at the rear of the barrel fillet.