In many respects, the TXb is order neutral. All pins in the same row are connected together. The pins with the white box around them are ground. The other two have pull-up applied (when powered).
That said, there is a standard ordering based on how the Teletype, Ansible, trilogy modules, TELEX modules, and Mannequins modules (Just Friends and W/) have things set up. Ground may be oriented differently depending on the module, but the order is the same.
The early ER-301 do not follow this ordering and swap the SCL and SDA lines.
You will find more info here in this lovely guide: