This looks like their marriage, giving her the surname of Hiles:

Surname First name(s) District Vol  Page
Marriages Mar 1872   (>99%)
Edwards John William Strand 1b 682   
Frost Elizabeth Annie Strand 1b 682   
Hiles Mary Elizabeth Strand 1b 682   
THOMAS Albert Strand 1b 682   
Thomas Alfred Strand 1b 682

The only way to be certain of this is to get hold of the birth certificate of one of their children.
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And her birth might be one of these:

Surname First name(s) District Vol  Page
Births Jun 1849   (>99%)
HILES Mary Eliza St James 1 112   
Births Sep 1851   (>99%)
Hiles Mary Elizabeth Shrewsbury 18 184

The 1881 census gives her birth as 1849, which is a near enough place, though I am not sure about the change from Eliiza to Elizabeth.