It's an embossed "@" also know as a "commercial at".
It has a lesser-known cousin, the "commercial each", a circled "e".
This now rather lonely character has been adopted by IBM as a logo for
e-commerce.
What is a "colophon"? Well strictly speaking, it's either
a publisher's or printer's emblem or logo or
an inscription at the end of a printed work, traditionally giving information
like subject, name of printer or publisher, date and place of publication etc
I'm employing it in the more modern form where it also gives information about the tools
and methods used to create the work, acknowledgments and contact information
(I'm following Wired Magazine's example).
