During the NS10's 23-year life Yamaha manufactured a number of different versions (or perhaps just used a number of different logos):
The NS10 started life as a (not very successful) hi-fi speaker: and the original NS10M shipped with cloth grilles, hence the grille-mounting sockets you find in the corners of this model (absent from the NS10M Studio that followed).
NS10M: The original domestic hi-fi speaker designed for vertical orientation (its front panel logo reads correctly with the speaker mounted with tweeter above woofer). This is the speaker that was too bright for Bob Clearmountain, leading him to resort to tissue paper over the tweeters — although, of course, it had to be the right kind of tissue paper.
NS10M Studio: Some time after Yamaha got wind of the NS10M's popularity as a nearfield monitor (and around nine years after the original product launch) a version badged 'NS10M Studio' was produced. This version was designed for horizontal orientation (the logo and connection panel text were turned through 90 degrees), incorporated a redesigned tweeter and crossover to address the HF tonal balance issues, featured a more rugged cabinet design without grille-mounting sockets, and had improved connection terminals.
Others: Web searches on NS-10 or NS10 will reveal some variants. There are versions badged NS10M Pro, NS10MX, NS10MC, NS10MT, and a miniature version that was sold in a 5.1 home-theatre package called the NS10MM. I've been unable to establish whether the NS10M Pro and NS10MX offer anything different (my guess is that they don't, but if anybody out there knows anything about them I'd love to hear it), but the NS10MC appears to be an NS10M Studio with a front grille, and the NS10MT appears to be a magnetically shielded and vertically oriented NS10M Studio with symmetrically arranged drivers and, wait for it... a reflex port. Aaaargh!
There are also obviously NS10-inspired products out there, by which I mean nearfield monitors with black cabinets and white cones. In the absence of any independent technical appraisal I'd be very wary of purchasing one on the assumption that it will offer anything like the performance of the genuine article. If you really want a pair of NS10s, eBay is probably your only real option, and you should expect to pay anything up to £350 for a pair in good condition.
I stole all of the above from here: The Yamaha NS10 Story
This is a cool NS-10m info page as well: http://www.ns-10.net/
I have never A/B'd them but I am sure there is probably not a lot of difference in them. You either love them or hate them no matter which model of NS-10 it is.