I never knew that these existed, but sure enough, one came plugged into the LaserWriter I acquired today. It is essentially a 4-port standard RJ45 ethernet hub attached to an AAUI transceiver, making it a 5-port hub.

I thought it was incomplete, as it did not come with a power adapter, but it turns out that the external power supply is only necessary if it is not plugged into an AAUI port, from which ordinarily it draws power.

Has anyone else seen one of these before?