For Steel engineers - What is this for a device?

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What is this for a device shown on http://www.rcnkonstantynow.pl/forum/....php?f=24&t=36, which was used for manufacturing of segments of Konstantynow Radio Mast, which consists of a hole exactly of the same cross section as the mast segments? I suppose it is used for quality control, but does someone know more about what it is exactly and if such devices are still use.