My understanding is that Bangla Dhun was essentially written by Raviji for the occasion. It was based on a simple folk melody or bajan from Bangla Desh; therefore, it never was a "regular raga". My favorite has always been the Dhun from the 1967 Monterrey Pop. It was based on Rag Panchamse Ghara. I heard this one again in person in Tuscon Az in late 2000. Still mesmerizing!