What do the red
Manuscript: http://bdh-rd.bne.es/viewer.vm?id=12826&page=246 page 675
As stated in my other question, I've been making the manuscript into a font.
This is the "Versicle" character, Unicode 2123, "Versiculus", ℣ if your screen can display it.
It is also possible that you may come across the "Response" character, U+211F, "Responsum", ℟, although this is much more common in the Liturgy of the Hours than in the Mass. (You will note that you have the word on your sample page, only slightly abbreviated).