The "m" is simply the suffix letter, part of the serial number. German military Lugers (and other small arms) were numbered by each manufacturer starting at 1, going to 9999, then 1a to 9999a, 1b to 9999b; 6652m followed 6651m and preceded 6653m. At the start of each year, numbers rolled over to 1, so a full Luger description to be unique would include the manufacturer, the year, and the full serial number including the suffix letter. The full serial may appear only on the front of the grip frame, though it would normally be on the barrel as well.
Note that the Luger pictured has the military style numbering, but the commercial number positions. That, plus the different number font on the barrel and the crown/N commercial proof, indicate a reworked WWI military pistol, with a new 7.65mm barrel numbered to match the grip frame, and the other numbers removed and re-stamped in the commercial locations.