It could probably be the M4P's frameline.
If one upgrades his M2 finder to M6's, you'll see that the bottom line of the 35mm (and 28mm if it's 0.72x finder) frameline is not whole because that's the place where the meter indicator should be. If one wants the 35mm frameline to be perfect, the solution is to change the mask back to M4P, which is not quite reasonable if one wants to upgrade the finder with a perfect non-meter frameline.
Normally there are a few choices:
Upgrade to M3 finder, this is interesting, probably 50mm user's favourite.
Upgrade to M4 or M4-2 finder, not many people doing this.
Upgrade to M4P finder then probably plus an MP anti-flare upgrade, or directly upgrade to an MP 0.72x finder with a non-meter frame line. I see this quite often.
Of course upgrade to M6(TTL) without mask modification is not rare, though. There are 0.58x and 0.85x for choice, This is attractive!
When glass wearers upgrade the finder, they will probably also upgrade the eyepiece to M6's because it has rubber at the edge so that the glasses won't be scratched.
Edit: To see the difference between the M4P mask and M6 mask clearly, go to Leica's website and download the instruction of MP. There is the picture for the MP's finder, which is very similar to M6's. The picture of the frameline that OP has posted are the non-meter framelines, which is the same as M4P (the 0.72x one)