Dear Patrick,
In 1937 the editor and scholar Jivaram Kalidas Shastri published an augmented Bhagavadgita (
worldcat permalink). I've got a copy in a box somewhere. I went to Gondal in the early 90s (with Peter Schreiner), and met Jivaram's son, Ghanashyamji, and we discussed his father's work and legacy. The 1937 Bhagavadgita publication was based on a birchbark manuscript of the work that was - if I recall - given to Jivaram by someone in Varanasi. The MS still exists, and is today in the library of the Gujarat Ayurvedic University in Jamnagar, together with all of Jivaram's extensive MS collection. I visited Jamnagar on the same trip, and examined the Bhagavadgita MS that was behind the edition. It was transparently a forgery. That is to say, it was a modern production, physically speaking, and plainly so. The birchbark pages were made of pieces cut very squarely and glued together in a patchwork, and the handwriting was large and student-like. Since Jivaram was an experienced MS collector and editor, I am surprised that he took the object seriously as a new and important manuscript.
The publication claims to contain verses that appear in no other transmitted version of the Gita, and therefore to be a better and more true version of the sacred work. I haven't checked, but it is possible that your verse is in that version of the Gita.