It could be be protective film glue residue…. But from the angle of the pic, it also t looks like a bad application silkscreen of the M1 logo…. Very hard to tell.
Get some WD40, spray it on a microfiber, rub the very end section where it’s in the black. It the WD40 removes it, then it’s just residual film glue…. Repeat process to remove all, then just wipe clean with a soapy solution and dry.