Gear coffers generally take the name of the metal from the corresponding tier of materials, but the level 78 coffers do not match their contents. They are labelled "High Mythril" coffers but the resulting gear is Dwarven Mythril.

Having searched the marketboard, the only gear using the term "high mythril" is a single set of level 44 tank armour.

If there is some issue with the length of the name, the level 78 coffers would be best termed "Dwarven" coffers as all the gear from that series is dwarf-themed.

Also, while less of an issue than the name, I have noticed that each type of coffer gets a unique description and the level 78 description ("a shining chest with dark metallic filigree") might work for this hypothetical high mythril set but sounds a little elegant for the dwarves.