\@M do in the definition of
\@makechapterhead in book class?
I took a quick look at
latex.ltx and couldn't find any explanation.
\m@ne stands for minus one, i.e. is a
count register with a fixed value of -1, which is used for efficiency reasons over a literal
\@M is defined using
\mathchardef and equal to
\mathchar"2710, i.e. 10000, and is used for the same reasons like above.
See the question \@ne \tw@ \thr@@ for more details on the reasons.
See also Documentation reference for LaTeX internal commands? for a list of further similar macros. I also can recommend you
texdef -t latex) to test for the definitions of such macros, e.g.
latexdef m@ne @M will give you the definition and numerical values of these two macros.
latex.ltx is the correct one.
At line 316 you find
At line 300 you find