I also thought it was SS, maybe SSO. It's a good suggestion but as far as I can tell she was a farmer's daughter in 1841 and again on the farm in 1861 then an annuitant in 1871. To be working in a Swankey Shop would be a bit odd, but hey, who knows!
William was living in Carlisle with a son in 1841 but then appears in Newcastle in 1851 where his other son was. Margaret is from Cumberland so there must be a connection somehow!

Bit stuck for ideas now, though I'm really looking for William's death record which is proving elusive!