A printer's mark, device, emblem or insignia was a symbol used as a trademark by early printers starting in the 15th century.
With the advent of printing, serialized publication came into use by printers and publishers.
But that made them vulnerable this week to work stoppages by pro-Government printers.
Dutch founts were in use by several printers in England at that time.
Although intended for internal use by printers and official bodies, essays sometimes find their way on to the philatelic market.
This suggests the signature was not in the original letter, but was an editorial choice by the copyists or printers.
I assume, that is, that they are errors by anonymous printers or copy editors.
However, the first securely dated book by Dutch printers is from 1471, long after Gutenberg.
Long strikes by metalworkers, printers and paperworkers forced management to accept this bargain.