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Yeah Steve, they sure do! That stuff is called 'skull', it builds up gradually over a period of weeks. 95% of the stuff they chisel off gets recycled.
[EDIT] Now that I had a closer look, I think you may be seeing the outline of what is called the 'saddle' - a T-shaped bulge on the top of the bottle, to facilitate the pouring-out of hot metal. Walthers neglected to include these on their models of these cars; a friend of mine sells saddles as an aftermarket add-on (resin castings).
Chears Ken, i could not for the life of me remember what the word was for the build up of slag,,, of course it's skull, i remember seeing it in MR a while ago