The Cantus Database is a wonderful resource!
A Link to the Cistercian antiphonal from the Abbey of Salzinnes, Namur, in the Diocese of Liège, 1554-1555 A.D.

Conditor Alme Siderum
http://salzinnes.simssa.ca/#z=3&n=5&i=1-006r.tif&y=427&x=143&q=conditor

Interesting how the melody of the Conditor Alme Siderum is almost identical to the one we sing today, only one melodic difference in the Augsberg and one in the Salzinnes. Also interesting is the different key used. Note also the difference in the Latin text!

N.B. The other Hymns Vox Clara / Verbum are not used as Advent Hymn in this manuscript.

I have been typesetting the above versions for use by our choirs, some of our members find it difficult to sing from ancient manuscripts that use abbreviated Latin text and changes of key position every other line! http://societyofstbede.wordpress.com/2014/11/30/the-first-sunday-of-advent/

It dawned on me after that perhaps the elision for "tuóque_amóre" was better represented thusly: