The form /das/ is neutral for time reference. In past contexts, it carries a past meaning and in present ones, it signals the present. The /das/ in Example 77 can indeed be replaced by /a/ as in the following example:
yu a chroo manyoor, yu beel pont, yu noo, aal difren taip a wok 'You used to manure the field, you used to bail punts, you know, all different types of work.' (Rickford 1987: 145, lines 327–328)
Source: Rickford 1987: 146 (line 338)
Source: Rickford 1987: 131 (line 253-4)
Source: Bickerton 1975: 67