MH= Mystery helmet, origins are slightly cloudy but it's beleived to be a direct recast of the ROTJ Boba Fett Helmet.
MSH1= Movie Sized helmet v.1 made by talented artist to more closely resemble the ESB Boba Fett helmet.
MSH2= Second version of the first helmet done by a different artist, highly accurate version of the ESB helmet.
For future reference there's info on all this stuff all over this site, all you need to do is a search. 
- Jorge