in the boxing ring at the start of the episode the south African flag can be clearly seen in the top right corner of the screen this flag was not adopted till 27 April 1994 , so is about 30 yrs too early to been seen.
The show is set in the 1960s, but the South African flag hanging in the boxing gym was not designed until 1994 when Apartheid (Segregation) came to an end, following the country's own Civil Rights Movement. Displaying the South African flag of the 1960s (orange, white and blue) was declared to constitute hate speech in 2019 by the South African Equality Court.
At around 1m:43s into the episode, Reverend Powell is in the boxing ring, promoting Doug Jones to the youth charity. In the background are numerous flags. One of the flags is the "Rainbow Flag" of South Africa, which didn't exist until March 1994, some 30 odd years before the events that this series takes place in.