This is the more recent version of the NTU at CatChat: Telecom Modem
This one has a sticker on it denoting it to be part of the DMS, or Digital Metropolitan Service, as does the line number format of NxxxxxxY (once again I note this is not my hand writing on the sticker).
DMS was a DDN service, it had all the advantages of being a DDN service (tighter SLAs remote testing etc) but was sold at a lower price as a response to other competitors entering the market, and Telstra needed to pretend the omgoing higher prices for other DDN services were justified.
The only limit for DMS was both ends of the service had to be in the same city, but this was a marketing limitation, not a technical one as DMS used exactly the same network as DDN.
One popular use for these services was lines out to bank branches and ATMes via an MJE - the MJE (Multipoint Junction Equipment?) allowed a single line from a mainframe to drive multiple services on a poll and response type basis. The MJE would switch each drop back to the main link when it saw a Status bit of 1 - the RTS (Request to Send) pin on the interface would raise the S bit on the DDN service, this would ultimately be mapped to RLSD (Receive Line Signal Detect - more commonly known as CD or Carrier Detect) at the far end of the service.