Base
Otto items are distinct in that all the bases of the items were covered with felt in a range of different colours.

Beneath the felt the clay is white, flat and without numbers.

Only the more modern items that are in production today have numbers on the base and are not always entirely covered with felt.

Stickers/Labels
Otto items are often found with labels, older items have a long thin paper sticker while more recent items have an oval gold foil label.