It is amazing that this came up today. I was searching for something else on the internet and came across this great website: Agora-Gallery.com There are many really good articles with excellent advice for artists. It is called “Advice from NY Art Experts”. They mention a COA in an article about documenting your art sale. I actually have been making these up for collectors of antique purses from my mother’s purse collection I have for sale. She was a well known author and historian of fine antique purses. I do think collectors are happy with the COA that comes with their purse and I think they have bought purses from me for just that reason. But I never thought of doing it for my own artwork!
You can look at this article here:(https://www.agora-gallery.com/advice/blog/2016/03/15/documenting-the-sale-of-your-artwork/)
They show an excellent example of a COA. 
A COA is an excellent idea and I am most certainly going to make up them up for my artwork from now on.
However, the article does not explain if you attach it to the back or not. I like the idea of attaching it to the stretcher bars or taping it to the back, if the painting is big enough for that.
Excellent question Paul ! Hope this helps someone.