My DM2s were purchased in 1973 and have the Coles supertweeter so I guess yours were made earlier.
Not sure if this helps, taken from Audiokarma site.
"The soft dome pictured on the DM2s is a Peerless DT10HFC found in Monitor Audio MA1, 4 & 5 mk 1, Mordaunt Short MS 235, smaller IMFs with the...