Did all metal Curta cases have an o-ring on the outside to seal the two halves when the case was closed? Were the o-rings always white?

Yes, there was a white o-ring on all metal cases. This o-ring often fades to an ivory color, becomes stiff, and even brittle.

Jack, thank you. When you service these units, do you place them if they are missing? I was simply going to buy a white o-ring of the proper size for my Model I.