Connection for C wire on Mastertrol Mark V zone control panel
It appears according to the manual for the Mastertrol Automatic Balancing System (MABS) EZ Zone (EZ-2 and EZ-4) Control Panels (PDF), that M1 is used for C.


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This does not appear to be the manual for your device, so this information may not be correct.
The manual for the MABS 2 Zone Panel (PDF), also shows M1 being used for C.


UPDATE:
After further review, you should be able to connect the C wire directly to the transformer terminals. Measure for Voltage between the T5 terminal of the zone you're working in, and the 1 and 2 terminals in the 24V 40VA Transformer section. One should measure 24 Volts, while the other measures 0 Volts. The one that gives you 24 Volts, should be able to be used as C.