She probably didn't "revert" to her maiden surname. It was very common and still is in rural areas today, to call a woman by her maiden name, in a familiar way, almost like the French tu and vous or the German du and sie. I have been through the poor relief claims for Cam'nethan and Dalziel on countless occasions and have noticed that the Inspectors seem to have known most of the claimants personally, as would be expected in such small parishes (as they were in the early decades of the 19th century) and my impression of the description, for example - "Peter Brown's wife ~ Ann Black" implies that Ann was seldom if ever referred to as "Mrs Brown". Even in widowhood she is called "the widow Brown ~ Ann Black".
The familiarity of the inspectors has sadly led to very little information being recorded in the early claims for these parishes. Parents names etc didn't have to be recorded as the inspector knew the details already!
Also, if Mary, in widowhood, was refused relief by Cam'nethan, it implies that neither she nor her late husband had the qualification necessary to be relieved by that parish. I think (someone will tell me otherwise I'm sure) that she would have been able to claim on her husband's residency in that parish if he had lived there for, I think, seven years. If residency could not be claimed the onus was on the claimant to provide proof of parish of origin for either herself or her late husband.
