Theory:

     Bepin Babu spoke quietly, his eyes still on the books.
 
     “Which month in ’58 are you talking about?”
 
     The man said, “October.”
 
     “No, sir,” said Bepin Babu. “I spent Puja in ’58 with a friend in Kanpur. You’re making a mistake. Good day.”
 
     But the man didn’t go, nor did he stop talking.
 
     “Very strange. One evening I had tea with you in a veranda of your bungalow. You spoke about your family. You said you had no children, and that you had lost your wife ten years ago. Your only brother had died insane, which is why you didn’t want to visit the mental hospital in Ranchi...”
 
     When Bepin Babu had paid for the books and was leaving the shop, the man was still looking at him in utter disbelief.
Explanation:
 
Bepin Babu proceeded to explore the bookshelves calmly for some time, indicating that he had dismissed Ghose's claims.

Suddenly, Bepin was reminded of something. He then asked him as to which month he was referring to about the Ranchi trip in 1958. Ghose replied that it was 'October.' After recalling the Kanpur trip in October 1958, Bepin Babu replied without hesitation. Bepin told Ghose that he was in Kanpur for puja with a friend in October 1958.
 
Kanpur is a large industrial city in the Indian state of Uttar Pradesh, in the north.
 
Bepin didn't accept Ghose's comments since he knew he remained with his friend throughout Durga Puja in October 1958.

Bepin then decided to end the conversation since he assumed Ghose had misidentified him for someone. Ghose, on the other hand, did not give up or leave. To establish Ghose's point and convince Bepin, he proceeded to talk about Bepin's personal facts. He reminded Bepin that one day at his Ranchi Bungalow, they had a conversation about Bepin's family during their tea time. He stated that Bepin was childless and that his wife died ten years ago. Also, since Bepin's brother died of mental illness, he has avoided going to the mental institution in Ranchi.
Reference:
National Council of Educational Research and Training (2008). Honeydew. Bepin Choudhury’s Lapse of Memory- Satyajit Ray (pp. 59-69). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.