Re. the Mary A Williams, born Woolsey (Devon), age 89 on the '91 Census, have you discounted the Mary Williams, died Dover, Sep Qtr 1893, Dover, 2a 649 - age 86
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Head of household, Joseph Newman, in 1891 is mentioned in the sale of
"No. 36, Seven Star Street contains 4 bedrooms, 2 sitting rooms and cellar, and is let to Mr Newman, a very old tenant, at 5s a week, tenant paying Water & Poor Rates".
Ages/years of birth are notoriously suspect on past documents. A sight of the death cert. might confirm this is the Mary A Williams at the 1891 Census address but would not identify her as the Mary A Williams you are chasing.