Posted by Howard Lewis on 10/02/2021 16:32:15:
My 1958 Zeus charts give 25 mm as being equal to 0,9843 inch. Dividing by 25 and multiplying by 25.4 gives 1.0000488. So The 25.4 mm to 1 inch figure that I have always used seems to be out by 488 millionths of an inch..
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Your 1958 Zeus was effectively superseded in 1959, Howard
Since then, the inch has been internationally defined as 25.4 mm
This is probably the best note I have seen on the matter: **LINK**
http://metricationmatters.com/docs/WhichInch.pdf
MichaelG.