That is a good question but, though not 100% positive, I would bet that the Carnelli stem is absolutely original with the exception of the saddle.

Most Botts to come my way have sported the Carnelli stem and this one even documents its presence. The Carnelli decalstiker is attached to the seat tube a few inches below the seat lug (you can see part of it in this picture and I just went to check to ensure that it is Carnelli.


Not sure if I still have this Bott and, though it is the same model, it sports the hex nutted stem and the Carnelli decal/sticker is in the exact same spot...


All in all, my guess is that the bike is mostly as issued, again the saddle being an obvious exception.