That is true of the Back to the future movies as well. In fact, I have a copy of My Bodyguard that has both full screen and letterboxed versions on opposite sides. And yes, the letterboxed version is nothing more than the "full screen" version with the top and bottom covered.
You can sometimes spot these types movies when watching the "full screen" version when you see an obvious microphone, etc. in the area that was blacked out for letterboxing and theaters.
People should also know that before about 1953 or 1954 movies were made in 1:37:1 which is just about the TV radio (1:33:1) therefore they don't need letterboxing. In the 1960s they re-releeased 'Gone with the Wind' to theaters and decided people wouldn't want to see a windowboxed (black bars on the sides instead of the top and bottom) movie. So they cropped it out to a 1:85 which had become the standard theaterical aspect ratio. It became a movie about Vivien Leigh's nostrils.
There are some phony "widescreen" films out there that apparently do nothing more than add the black bars to a fullscreen format. A Christmas Story supposedly does this...