The 'M' is the optional method of diaplay
Using figure 1 from the referenced page above, there are two formats for the HIN in boats made prior to Aug 1, 1974.
The first,, ABC 12345 1272
where ABC is the manufacturer ID code
where 12345 is the serial number
where 1272 is the date of certification.
There is no M involved.
The alternate version is ABC 12345 M73E
where ABC is the manufacturer ID code
where 12345 is the serial number
where M denotes the optional date of certification method
where 73 is the year of manufacture
where E is the month of manufacture based on the table:
AUG A FEB G
SEP B MAR H
OCT C APR I
NOV D MAY J
DEC E JUN K
JAN F JUL L
This is lifted from
http://www.boatsafe.com/nauticalknowhow/hin.htm.
So the M means use the alternate / optional method.
Hope this helps