Don I would think the serial plate would be in the same place on the M-2 as is on your M-4... Engine side of firewall..you should be able to locate the 4 small holes (maybe 2) that held the plate. The only other place that the id tags were mounted was inside the car, passenger side, toe board.
If you know the serial number (from registration?) hopefully with build suffix (C- California, L- Lansing, T- Toronto) I think Frank Witkowski is still selling reproduction plates that can be stamped to match your numbers.
The only place that the serial numbers are located is on the id tag
That's this car lot 130
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Don't think it had a vin plate. The rad crest is the 1928 model year style of blue D on white background, so M2 April 1 1928 to July 31 1928.
Any chance MOT / DOT ? in California can supply info as to vin off the 1951 Calif license plates ownership 5X 4 482. Just a thought seeing as you have that much.
Another thought is engine serial plate don't they show what plant the car was built at ? That would give you the C, E or T vin suffix as M2 not built in Lansing.
Once the vin / serials etc are known we can add this M2 to the 1928 Durant's in the Registry.
Where Are You From? Ottawa Ont Canada
Do You own a car built by Durant? 1932 614 sedan made by Dominion Motors