The old brand on the backside I have done to many a cow and calf and I can assure you that you are very lucky that it ended up healing over and was a permanent marking. This isn't the first time I have heard of it happening like even Jay mentioned too. Those old "radiating" heaters can do some damaged to the unsuspecting. I grew up in an old house that had those in a couple rooms and learned real quickly just how hot they can get. That fraction of second for the brain to register pain from the rear was just long enough or short enough in this case to save your arse so to say.