Type 'M' autoalarm courtesy Museum of  History of Science.

There were two versions of the Type M. The earlier was merely referred to as the Type M and from other, approval documentation dated from the early 1930s (1932/33). However, in the late 1930s the Receiver was redesigned employing KTW63 beam tetrodes that were first marketed in 1937/8. The name 'Vigilant'  was then added to the device at that stage (apparently). Oxford's Museum of Science holds a Marconi technical manual with the title merely stating 'Type M' whereas the current manual is  titled 'Vigilant Type M'.

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Contributors and Credits:

1) Type D photo      ttps://www.mhs.ox.ac.uk/collections/ search/displayrecord/?mode=displaymixed&module=ecatalogue&invnumber=24207&irn=15961&query=
 

Apr 13/26