The highlighter is the copula da.
In general there is no element separating the clefted constituent from the rest of the clause.
Occasionally (particularly in Weygandt 1798 and Schumann 1781) the relativizer disi appears in cleft-like constructions; such cases can sometimes but not always receive an alternative analysis involving a true relative clause.
Source: van Dyk ca. 1765: 29
Source: Schumann 1783: 19