Puzzle solutions

For anyone that's stuck or would prefer not to solve the puzzles included in the game, below are all of the solutions. Don't read if you don't want anything spoiled!

Solutions:

The Bookshop - Antony and Cleopatra
(the letter refers to a quote from that play: "Age cannot wither her, nor custom stale / Her infinite variety". The significance? In the Good Omens universe, that line's inspired by Crowley who says it while watching Hamlet with Aziraphale)

Crowley's flat - 1967
(this time, the answer is in the video that plays on Crowley's TV. It's stuck on the scene from 1967, when Aziraphale give Crowley the Holy Water and that's why Crowley's safe is now set to that number)

The Ritz - 1976
(this one may be a little tricky to figure out, but the answer is in the music that Crowley and Aziraphale hear in the background, while the player only hears a piece of it. That song is "Good Old Fashioned Lover Boy" by Queen and it was recorded and released in 1976. The lyrics to that song are scattered all over the scene and once you put in the correct year, Crowley will explain why that song in particular)