Owning the Mega Man Robot Master Field Guide, I've encountered this myself as well (it is also mentioned in some the the manga translated in the more recent years, worth a read btw) And some robots have other numbers (the gameboy "Mega Man Killers" use the MKN designation, Bass/Forte uses SWN, the original robots from Mega/Rockman & Bass/Forte built by King use KGN - which excludes the robots re-used from MM8 - , the robots trio based on chinese folklore from Wily Wars ie: Busterrod G use WWN, though the Dark Man robots (from NES MM 5) don't have numbers to them, and neither do several support robots (ie: Beat, Auto, etc) nor does the Doc Master K-176 from NES MM2 and Duo from MM 8 onwards. The Fake Man (Police Robot from MM 9) also lacks a number while the rest of MM 9's Robot Masters have the DLN for being Light Robots (though this is glossed over by some people for whatever reason). The Stardroids from GB MM 5 have SRN for their numbers except for Sunstar himself. Another note is that King himself lacks a number despite Wily being credited as his creator.

I'm done rambling now. Being sick does that to me. *blark*