When looking up the leFH18 cannon on Wikipedia, the article mentioned that the leFH18M version was the original with a muzzle brake added, and the later leFH18/40 had a more efficient muzzle brake. I didn't know about the two types until I saw this, but unfortunately they didn't show pictures of the muzzle brakes. Did they look the same, or different, and which is the one on the AFV Club kit?

The don't look the same. The 18M brake is sort of simple. It flares out to a larger tube shape and has 2 holes on each side. The 18/40 brake has vertical curved plates through it. You can see the latter over at Lucky Model's (or Hobby Link Japan's) website. Look up "lefh" with brand set to AFV Club and you will see the /40 and M kits listed. The /40 kit has a view of the muzzle brake. AFV Club made (makes?) kits of all 3 versions.