His bed was probably wooden and decorated with iron. Another possibility is that the Hebrew word refers to a sarcophagus (a stone coffin). In that case, the object may have been a burial container rather than an ordinary bed.
Rabbah is the same city as Rabbath Ammon (see study note on Deuteronomy 2:19). The text suggests that this object could still be seen there when Deuteronomy was written.
This may mean that some kind of public display existed in Rabbah. People could look at this artifact as a reminder of the past.