Chart Datum at Avonmouth sits 6.50 m below Ordnance Datum Newlyn. Here is the arithmetic that goes wrong when a height file does not name its zero.
Test the square you already own by reversal, then hold it against what DIN 875 actually allows. Eight squares, my own measurements, one of them firewood.
A 1943 South Bend that showed up 78 tenths hollow at the headstock end. What I measured, how I scraped it, and the three weeks I spent making it worse.
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