This thread is for links to any postal agency providing
any information regarding the printers,print method,
engravers, designers, colours used, gum and paper type
etc.
Whether for past or current issues, online or available
via snail mail.
The best in my opinion is from the
USPS.
Their Postal Bulletins provide not only all of the above
but even the manufacturer of the particular printing press
used.
Here is a list back to 2001
https://about.usps.com/postal-bulletin/welcome.htm