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<p>[QUOTE=&quot;proofartoncircs, post: 961461, member: 25952&quot;]&lt;&lt;Just curious if you know why they chose mod 6 to determine the check digit? I tend to use luhn algorithms because of their simplicity to check, I can&#039;t image many people can divide by 6 in their head...&gt;&gt; </p><p>&nbsp;</p><p>I don&#039;t know if much thought went into it.&nbsp; The sheets were 12 subjects in the old days.&nbsp; I believe they were cut in half before numbering.&nbsp; THe top note of the first half sheet would be number 1 which would be chaeck letter A or G.</p><p>&nbsp;</p><p>Now, surprise, everybody!</p><p>There was an X prefix for experimental notes.&nbsp; That was the X B block for series 1928 and 1928A silver certificates.&nbsp; Y B was also an experiment and Z B was the control.&nbsp; !0,728,000 X B notes were printed and are worth a premium today.[/QUOTE]</p><p><br /></p>

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