| http://www.w3.org/ns/prov#value | - As can be seen from the drawings, the plate-shaped mask element 60, or a lower part 26' of center mask 26, can be releasably connected with the cylinder-shaped housing or the cooling housing 61, as by a threaded shank, so that the plate-shaped mask element 60 can be replaced at any time, for example if it has been coated with aluminum.
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