hi there, i can help you with the answer..
i currently have 112K and 112M with me..
the bezel spacing on A is different, with M being wider..
the spacing on B is exactly the same..
not only the bezel are different, there are other differences i've noticed(feel free for anyone to copy paste and create a new topic if the info is good enough to share, don't forget to credit):
for illustration purposes,
here's my 112K:
DSC_9315 (1) by loelianbao, on Flickr
and here's an exact same sample of my 112M:(there are some other same sample such as item number 300502613742)
http://cgi.ebay.com/PANERAI-PAM112-M-LUMINOR-BASE-44MM-PAM-112-BRAND-NEW-/400195383967?pt=Wristwatches&hash=item5d2d80f29f
the most noticeable one is the hands, compare the hands and you will see 112M has pre-V style hands with longer lume marker on the tips, and the K has normal modern lume marker, not all M series has this, but some samples does, and not sure if it's only M specific series or it's also happens on the previous series.
next is the crown guard lever, on the 112M it is thinner, you can see the difference on the notch holding the crown.., another clear sample of K series thicker CG lever, last two pictures of the listing:
http://cgi.ebay.com/Panerai-Luminor-Base-Historic-PAM-112-K-44mm-Watch-/110647512062?pt=Wristwatches&hash=item19c31b0bfe
still on the CG difference, the notch for finger to pull(less noticeable).., the roller ball is highly polished on the M, and brushed on the K, nicer on the K as it's almost invisible to the eye.
other differences:
less noticeable from just pictures and could be seen easier in person, is the dial black paint are different, color is the same, but 112K have thicker paint with emboss effect on the circle area of the 6, 9, or the curved area of 12.., the sandwich has an embossed effect which is beautiful.. while the 112M paint on the dial seems flat.
lastly on CG, the crown on M seems fatter because the squared hollow spacing for crown is smaller. this is also very miniscule and not noticeable.
onto the crystal.. the only difference is AR. on M serie is more blue/neutral, while the K serie i have has amber-gray-ish tint on AR, it's very hard to describe other than cooler on M, hotter on K.
personal rant: i prefer the K, bought the M for the very nice serial number, a triplet.. and originally planned to let the K go.. now i'm not so sure anymore.. and want to keep both but i don't think that's possible..