Are you completely positive there is no "O" in front of the serial? Have you tried scratching a bit with the nail or fingertip in case it is hidden or almost invisible?

I don't think different letters were assigned to different markets: one of my IIIR is marked in meters, while the other is marked in feet, and both share the same serial structure...

The reported IIS serial number starts with one single "H", too. I'm waiting for the serial of the IIS from another colleague that should confirm if it may be a typo or it is common to this particular version.

Could it be possible for you to upload a bigger image of your camera?