Useful websites
Alphabetical Listing (click to jump to subheading in a new window)
General USA
Early Postage Dues
http://www.apnss.org/PostageDues.htm
Post 1900s USA
https://www.pnc3.com/primer/primer002.htm
General USA
http://www.shaulisstamps.com/tips/WetVersusDry.htm
Canal Zone
http://www.eworldstamps.com/canal-zone-dry-versus-wet-printings-1928-1940/