Just to note the address in 1851 & 1861 don;t actually have house numbers. The number on the left of the image is just an entry number
Have found a death notice - on 12 A8gust 1867, Griffith Williams, cabinetmaker, of 11 Segontium Terrace
1865
To be Sold by Private Treaty,
FREEHOLD HOUSE, situated in 11, Segontium Terrace, Carnarvon, containing two fine Kitchens, one Pantry, and Scullery, two Parlours, one large Sitting Room, one Double Bedroom, two Single ditto, thre fine rooms Attics. AIso, a YARD, with a DWELLING-ROOM attached, now occupied by the Cwmdyle Copper Mine Co, Apply to Mrs. GRIFFITH on the premises
Suggests they didn't live there that long before he died