Goofs

The Invitation Cards of Leela Basu & Atanu Chandra Sen and Byomkesh Bakshy & Satyawanti Devi Ghosh has most of the things similar, for example, brother and sister names (Manu, Govind, Ila, Babu, Lata). However the year mentioned (1350) is from the Bengali solar calendar, which coincides with Gregorian 1942.

When Dr. Guha return and Byomkesh meets him on the roof of the building at the time there was a blackout Kolkata as Japanese were bombing the city, but behind Bymokesh a light bulb can been seen, which is switched on.

When Byomkesh sends Ajit to deliver a letter to Dr. Guha, Ajit reads it in car and Byomkesh's voice narrates the letter in background. But the content of letter doesn't match with the voice narration.

When bombing is seen in the distance, there are no searchlights, zeppelins or anti-aircraft explosions visible at all. Even from that distance, some of the defenses should have been visible given the scale of the bombing area and the light emanating from it.

The film takes place in 1942/1943, but one character is shown reading Inside Detective magazine from July 1946.

An openly practicing Japanese dentist, language teacher and samurai swordsmanship enthusiast (and also a secret service agent) in a British colony during war time would never be allowed be stay free under any circumstances. He would be detained in an internment camp of which many existed in India during WW2 and held German and Italian nationals. In the movie Dr. Watanabe is neither disguised as any other national (Chinese, Korean, etc.) nor is he under British secret service surveillance even assuming they deliberately allowed him to stay free.