Dear Group,
A person that I'm researching was born in Kaminiec Podolski to a Jewish family in 1884, and was given the name Bronisław (given name).  
Throughout his life, he used this name, until he perished in the Holocaust .

Recently, I found one Polish government source (1920), where the given name Mieczysław was used to identify a person who I believe is same one I know as Bronislaw.

I would like to ask native Polish speakers if these two names are known to be interchangeable.

Thank you very much for your help - Yaron Wolfsthal