On page 9, you can kinda see Yuu playing with a spiral made out of wire. Idk if this is significant, but it looked a lot like the keychain of the key for their kettenkrad.
Oh, also on page 19, we get a close-up of the key and key chain while the girls are talking about the past and the feeling of nostalgia. If the wire in the past (on page 9) really is the same one as the current one, then that’s pretty great symbolism made by the author.