In the Dortmunder novels by Donald E. Westlake, Kelp always steals cars owned by medical doctors. In the movie, the license plates of the stolen cars have the letters "MD" as part of the plate number. (Kelp's reason is that doctors have expensive tastes, and their cars are usually loaded with the best options - leather seats, stereo, etc..)
Absurdly, the film's title was changed on its initial British release to "How To Steal A Diamond In Four Uneasy Lessons", on the grounds that people might think it was a rock-music concert film like "Woodstock". This was much disparaged at the time by journalists and members of the public, and the film has reverted to its slicker original American title for all its British TV showings and for its video and DVD releases in the UK.
The weapon they are carrying during the helicopter ride to and the assault on the Police Precinct is the Swedish 9mm SMG m/45. Developed during WWII. Still in use within the Swedish Armed Forces (nowdays in a very small number). Once used by American Special Forces because of its simplicity and reliability. Also manufactured in Egypt on license.
When this was first shown on Britain's ITV in the Autumn of 1976, it went under its original title, "The Hot Rock'. After getting a UK cinema release under the title" How to Steal a Diamond in 4 Uneasy Lessons ", TV viewers thought they were getting a brand new Robert Redford movie, and there was great consternation when they realised it was the same movie.