Maybe the actors could not handle a real motorcycle with racing ancestry!
Or the producers were afraid to have the actors crash a really valuable motorcycle?
Or maybe they could not afford one because Nortons nowadays all seem to have Brough Superior gearboxes which cost a fortune thus eceeding the film budget!
We can go on for ages I guess.
I do wonder how they fixed the additional fork springs!
All joking aside, it is a really odd think to do. I haven't seen the film, or know who or what it's about but the only reason you'd disguise a BSA as a Norton is if it is crucial to the story line or historic accuracy. Like Che Guevara on a Norton in the Motorcycle diaries.
The film is about L Ron Hubbard, founder of the Church of Scientology.
The scene called for a Norton 16H & they tend to use one main props supplier so maybe they only had an M20 & hastily put the Norton parts on it at short notice.
Springs look to me like they are bolted to the headlamp mounts at the top & made up brackets lower down & maybe not "active".
Recently i saw a terrible example of fakery in "Knight & Day" where Tom cruise (also a Scientology fan) rides a 1000cc Ducati & lots of the shots are him on a heavily disguised 500cc aprilla with "Ducati styling".
Hubbard was a bike enthusiast & evidently rode the 16H but he also actually did own & ride a BSA pictured :-)
I went see this film yesterday. There is not much bike /WM20 content, but I enjoyed it and think a quiet a few on this forum will make some sort of identification with the main character Freddie Quell