The frame of the altarpiece is largely lost, but an idea of its appearance is provided by the scene of 'Saint Nicholas ordained as bishop of Myra', which shows a tripartite altarpiece on top of an altar covered with a red cloth (or antependium) decorated with an orphrey. Interestingly enough, the width of the altarpiece's side compartments, without their frames, corresponds to that of the painted surface of the narrative scenes (ca. 41.5 cm).

Two minor details in the scene of the 'Ordination of Saint Nicholas' also suggest that the two pieces once formed an ensemble. The altarpiece depicted in the painting shows Saint John the Evangelist to the left and Saint John the Baptist to the right of the Virgin, the very same Saints represented in the pinnacles of the actual altarpiece. Moreover, the two bishops to the left and right in the scene seem to echo Saints Nicholas and Proculus on the real altarpiece.

There is one anomaly, however. Painted antependia usually consist of horizontally joined planks, but in this case the narrative scenes are painted on vertical planks. Dowel holes on the right side of one panel, and on the left of the other, indicate that they were joined to a central part now lost. Presumably the whole piece was conditioned by a tripartite structure, similar to that of a polyptych, in which each compartment consisted of a separate panel. Some other works in Ambrogio's œuvre have a similarly singular structure, such as the triptych from Badia a Rofeno (now in Asciano at the Museo Civico Archeologico e d'Arte Sacra Palazzo Corboli) or the polyptych probably intended for the convent of Santa Maria Maddalena fuori Porta Tufi in Siena (whose remaining parts are in the Pinacoteca Nazionale).