My understanding is that when Continental took over from National they did in fact make master dies with secret marks for the high denomination banknotes – 24c, 30c and 90c, but they did not use these to make new plates.
I can reference this link concerning the 15 cent 'betwixt-and-between' 15c: http://www.jamesdire.net/sm15bn.html
. Bill Weiss also wrote a monograph on this stamp.
My questions, please: Is this information correct? If so, what is the primary source for the above article? Also, if so, were die proofs made – showing the secret marks – of the 24c, 30c and 90c?
I understand the only known Continental 24c is a ribbed paper. Has it sold since the Siegel 2004? https://siegelauctions.com/lots.php...8&page_no=11
also details history and recent findings.