Back in the early days of digital telephony (1975 SL-1 Release) with Nortel, the only digital telephones available were the Q Series Wedge sets that resembled a large army tank. Later in the Mid 1980s, the wedge sets were redesigned to more lower profile sleek sets with re-engineered aero handsets and renamed “Meridian” SL-1 digital sets and the familiar Meridian Grey button caps, RLS Orange Button and Red Hold Button, so familiar with the Norstar and Meridian 1 phones were introduced. There was another model similar called the M1009 and M1209, one of which contained an add on display module that showed 10 RED LED numbers that could display incoming CLID, very primitive at the time.
So I would call these the second generation digital sets from Nortel. First came the Q Series SL-1 Electronic/Digital Wedge sets, then these M1XXX sets described here, then the Delta SL-1 Sets M2000, then the Aries I M2000, then the Aries II M2000 and finally the last generation and still being manufactured Taurus M3900 sets.