Yes, the attenuator is the main difference along with some other tweaks, there is good info available from the builder hereWhat's the difference between Mk1 and Mk2?
Keep in mind the MKII comes in at about 40lbs. They have the rep as very lightweight and the original specs were published as 36lbs. My MKI is 35 lbs with the original speaker and guys talk about them as under 30 so be really careful and request an actual weight on anything you are serious about if lightweight is an important consideration (as it is for me). For some reason, lightweight amps get 5-10 lbs shaved off of them when guys are raving about them online and then that becomes circular reference. Alessandro Rottweilers are another example of this happening.
Neo speakers are great these days and the Emi Lil Texas (american) or Creamback neo (brit) are the easiest ways to remove 4 or more lbs from just about any 12" single speaker combo, imho.