You are right, this is Jean Schlumberger for Tiffany Ibex, probably a custom order done by T&Co. The workmanship is superb & intricate, i have scrutinised every single details including the bail & joining parts with my 10x & 20x Jeweler Loupe. Unlikely to be a converted piece as there will be tell-tale signs & markings no matter how skillful the jeweler is. Of coz this is my personal opinon & perspective. Thanks for sharing.
if you take it into Tiffany's to have it appraised (I think its $100), they will be able to tell when it was made and for whom.