With Early Access, the Special Assignment: Titan Resurgence world quest will expire in 6 hours on NA servers. This rewards a Restored Coffer Delve key, and there is no way to get this one back once it expires!
There are only a couple ways to get Restored Coffer Key for Delves.
In Early Access, World Events are not giving rewards and thus, the only way to get Restored Coffer Key in Early Access is Special Assignments. Special Assignments last 1 week each, with one rotating on the weekly reset and one rotating halfway through the week.
As such, Early Access players can get 2 extra Restored Coffer Key. One from the Special Assignment: Titanic Resurgence that expires in a couple hours and one from the special assignment that expires when the Expansion officially launches.
There is no way to get these keys back. Complete the World Quest before it expires, or be 'behind' in Delve keys.
As a reminder, in order to unlock World Quests, you need to complete the Main Story Quest (MSQ).
Restored Coffer Key is used to open big chests in Bountiful Delves. Right now you can only get 571 gear from using them, but once Tier 8 unlocks with Season 1, you get get 603 gear — so Restored Coffer Key are very important for gearing up! For more information on Delves, click here.
Quick tip for Special Assignment: Titanic Resurgence, you damage to boss by using Skyward Ascent (Default: #2).