Recently, Rajat Maheshwari from Indore, India asked this question:
"When does Raj Kapoor introduce himself as Ranbir Raj in a movie?"
[Note: This is interesting because it is his own name in real life.]
I vaguely remembered that it was in Shree 420 (1955) and said so. Later I verified that it was correct.
Here is a page from my copy of the HFGK Volume 3 and a short clip from the film. By the way, Nargis father is Ramesh Sinha.
Now You Know!