... sad.. When in doubt read the manual!
Same for my friend who keeps asking me what the buttons do on his car (that I have a similar one of) I say read the manual!!
My dad taught me about those switches on an old Ansco Regent he gave me to use in 1979.
You will see the "M" would have worked if you had the shutter set to perhaps 1/30 or for sure 1/15. Do the test mentioned above.