Board Thread:Off-topic Discussion/@comment-27713488-20171119055244/@comment-27713488-20180411215147

It's 4 "Low Level Bags" obtained in the Exploration Guild if your average party level is 19 or less. Also the Steam version gets an achievement.

Low Level Bag is an accessory rather than a usable item, and gives 20 ATK, 30 DEF, 20 SPI, 40 AGI, element guard, immunity to poison/blind/silence, and double EXP gain (which I believe is unique to this item).