Hey Tob, as far as I know the difference is the M version was designed for horizontal operation to better serve the meter bridge of a console. There is a link in one of the posts here where the designer explains the evolution of the NS-10 to the M version.
The post was essentially about the NS-10, maybe 6 months to a year back. Sorry, I don't have a link, but if you search Google, on Yamaha Corporation History, it may give you a link to the article.
They use different tweeters also. The newer NS10m uses a tweet with a square edged grill while the original NS10 has a rounded grill. The newer tweets are available for replacement while the older rounded tweets are out of production. Kurt