What’s My Line? is a panel game show that originally ran in the United States on the CBS Television Network from 1950 to 1967, with several international versions and subsequent U.S. revivals. The game requires celebrity panelists to question a contestant in order to determine his or her occupation, i.e., “line [of work],” with panelists being called on to identify a weekly celebrity “mystery guest” with specificity.....
https://en.wikipedia.org/wiki/What%27s_My_Line%3F
You can watch (original segments) on youtube. It’s hilarious to see some of the ‘mystery’ guests.
I am remembering. They wore blindfolds for the M guests. I won’t say what size B&W I watched it on.