Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Second Ecumenical Council:  The First Council of Constantinople.: Introduction on the Number of the Canons.

p. 171 Introduction on the Number of the Canons.

(Hefele, History of the Councils, Vol. II., p. 351.)

The number of canons drawn up by this synod is doubtful.  The old Greek codices and the Greek commentators of the Middle Ages, Zonaras and Balsamon, enumerate seven; the old Latin translations—viz. the Prisca, those by Dionysius Exiguus and Isidore, as well as the Codex of Luna—only recognize the first four canons of the Greek text, and the fact that they agree in this point is the more important as they are wholly independent of each other, and divide and arrange those canons of Constantinople which they do acknowledge quite differently.

Because, however, in the Prisca the canons of Constantinople are only placed after those of the fourth General Council, the Ballerini brothers conclude that they were not contained at all in the oldest Greek collections of canons, and were inserted after the Council of Chalcedon.  But it was at this very Council of Chalcedon that the first three canons of Constantinople were read out word for word.  As however, they were not separately numbered, but were there read under the general title of Synodicon Synodi Secundæ, Fuchs concluded they were not originally in the form in which we now possess them, but, without being divided into numbers, formed a larger and unbroken decree, the contents of which were divided by later copyists and translators into several different canons.  And hence the very different divisions of these canons in the Prisca, Dionysius, and Isidore may be explained.  The fact, however, that the old Latin translations all agree in only giving the first four canons of the Greek text, seems to show that the oldest Greek manuscripts, from which those translations were made, did not contain the fifth, sixth, and seventh, and that these last did not properly belong to this Synod, but were later additions.  To this must be added that the old Greek Church-historians, in speaking of the affairs of the second General Council, only mention those points which are contained in the first four canons, and say nothing of what, according to the fifth, sixth, and seventh canons, had also been decided at Constantinople.  At the very least, the seventh canon cannot have emanated from this Council, since in the sixth century John Scholasticus did not receive it into his collection, although he adopted the fifth and sixth.  It is also missing in many other collections; and in treating specially of this canon further on, we shall endeavour to show the time and manner of its origin.  But the fifth and sixth canons probably belong to the Synod of Constantinople of the following year, as Beveridge, the Ballerini, and others conjectured.  The Greek scholiasts, Zonaras and Balsamon, and later on Tillemont, Beveridge, Van Espen and Herbst, have given more or less detailed commentaries on all these canons.