The "Nebr." and "Kans." overprints are not considered precancels, but are considered a Regular Issue and are listed in the Scott Catalog from 658 to 679. According to the Scott catalog:
"This special issue was authorized as a measure of preventing losses from post office burglaries. Approximately a year's supply was printed and issued to postmasters. The P.O. Dept. found it desirable to discountinue the State overprinted stamps after the initial supply was used."
Interestingly, the "Nebr." and "Kans." overprints do have precancel varieties as well. Some are shown at this link:
http://www.apnss.org/KansNebr.htmThe "Canal Zone" overprint shown is listed under "Canal Zone" in Scott Specialized as Scott #118 issued on September 1, 1939. 1,030,000 copies were issued. Check out Page 26 of this link for more info. on your example:
http://www.canalzonestudygroup.com/...irstEXH3.pdf