There are various versions of this contraption around, but this one was the cheapest my welding supplier had. Basically it is a bit of sheet-metal with the corners nibbed off. You would use this tool or some other contraption to 'inspect' your welds for correct size.
E.g. 1/4 fillet really is a 1/4 fillet . This 'inspection' tool is for me a training aide-mostly a timing thing.
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