O Lamm Gottes

The intended structure of Kauffmann's prelude to "O Lamm Gottes" is not immediately clear. Kauffmann gives an alternative ending for the piece that can be played from bar 9 and further. And the alternative has a second part, called "a 3 per Imitationes". Did Kauffmann intend this piece as a da capo prelude with the imitationes as a middle part? Or are they two different pieces: one that ends as a four voice piece, one that ends with the imitationes? Or are they intented as three seperate pieces? I don't know. In the performance I choose to play it as a da capo prelude.

Score available here: http://partitura.org/index.php/georg-friedrich-kauffmann-o-lamm-gottes