In the school hallway scene where Tarby is being invited to Lewis's house, a boy with glasses and a brown striped shirt passes them by on their right side and about two seconds later, you will see the same boy passing them by on their left side.
Jonathan and Lewis both have the surname 'Barnavelt'. Lewis should have had a different surname, that is, his father's surname and not his mother's (who is Jonathan's sister).
Florence uses the phrase "24-7", a saying that didn't come into use until the mid-1980s.
The Backpack that Lewis is carrying while travelling and to and from school is a Yugoslav People's Army Combat standard combat backpack which was produced from 1975 until the 1990s. It was extensively used by each side during the Yugoslavian wars. It was designed around 1975 and was not available in 1955. There are several varieties. (black leather straps instead of leather colored and so on) It seems like the front shoulder pads were removed, but it is definitely a M75 combat backpack of the YPA.
Both Lewis and his mother use the word "anyways". This incorrect, distorted pronunciation of the word "anyway" did not become widely popular until recent decades.