A 'purchased' grave is one where, when the first burial happens the family paid for rights of further burial in that grave - so that later, other family members could be buried there (though the number of burials in one grave would be limited) and a grave stone could be erected.
As the burial register shows that Sarah is in Ward 12, Section A, grave 1721 and Alfonso is in the same Ward and Section but in grave 1776, its fairly safe to assume that (unsurprisingly given their ages when they died and the fact they left children) the extra money to purchase the right of future burials wasn't available.
As such it is 'extremely' unlikely that there will be a headstone for either of them.
Boo