Also be aware that the first three Canal Zone issues - Scott nos. 1, 2, and 3 - were heavily forged right from the date of issue. Given the high catalogue values of these issues ($650, $300, and $400 respectively for unused hinged copies), they should be certified, preferably before or definitely after, purchase. I just had a full set come back from PSE certified as genuine Panamanian stamps but with faked overprints.

Scott numbers 15 (CV = $2,000) and 47 (CV = $2,750) should likewise certified.
Scott numbers 46-47-48 and 67 are often hard to identify and certificates are a good idea.