Not so sure if it really is very rare (as a matter of fact if anything really is for the Ustaads at HF) but I have never heard the original from Mahendra Kapoor. I remember my eldest brother singing it to me often when I was 4-5 years old and today I suddenly realized that I can actually seek help from my older brothers (and sisters) at HF to help me hear it again. The song goes as "Samay Ka Panchhi Udata Jaye, Ek Ujala Ek Kaala Par Phailaye". This is from film Aag (the one from 1967).
Anyone ?