Why do Erowa chucks have an O-ring at the bottom of the drawbar bore while Maxx-ER chucks do not?

The bottom O-ring in the Erowa mechanical chuck serves a through-flushing function. The corresponding Maxx-ER (Erowa OEM 100% cross-compatible) mechanical chuck does not use that bottom seal.

Use the required O-ring on the spigot. The mechanical locking mechanism provides the retention force; the bottom flushing seal does not generate clamping force.