We can speculate based on the seemingly default values that these bats were the result of an item server glitch or exploit.

The defindex is 0. All TF2 items have a unique index number called the defindex, and for Bats, the defindex is 0.

The quality is 0. All item qualities have a numerical ID, and the ID for Normal quality is 0.

The level is 0. All items in TF2 can have a level from 0-255, and the Normal Bats are all level 0.

The origin is 0. All items in TF2 have a numerical origin ID, and for Normal Bats, it is always 0 (Timed-Drop).

There are '5' Level 0 and '9' Level 1 Normal Bats in existence.

Source