The "c" means a sub-division to M5/8 to manufacturer these items.
One can also find the numbers with an "a", "b" and "d". I do not
have the time to look for the exact meaning for the letter-codes.
Since about 1938 the F.W. Assmann-concern held the numbers
M5/8, M5/8/1 (for buttons for political leaders) and M5/8/2 (for
HJ/DJ Karibinerhaken according to DRGM Nr. 1323713). It might
be obvious the shown carabine-hook is manufactured before the
introduction of the code M5/8/2.