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<pre wrap="">Hallo David, and thank you for your suggestions. I have now found the following :
1) The primary cause of the displacement is the PDF/X-1A:2003 specials -- remove these, and the page returns to its normal vertical position, but shifted outwards (leftwards) somewhat;
2) The use of \usepackage {color} within e-plain's \beginpackages ... \endpackages; in TeX Live 2016, this makes the page size considerably wider, IFF the PDF/X-1A specials are not emitted.
The displacement is visible both in TeXworks viewer and in Adobe Acrobat.
A4 is indeed the default in both, and the difference between A4 and letter is only 3/4", whereas it appears to require a 1" correction ...
** Phil.
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