Pointers For Canal Zone Overprint Fakes?

I'm looking for pointers on how to tell fake Canal Zone overprints.

Here are some of what I am looking at:

The ones to be really concerned with are Canal Zone numbers 1, 2, and 3, as they were heavily faked during their very short period of use. Always buy with a certificate, or if you are sending it in after purchase, get a guarantee of a refund + cert costs if it comes back as fake.

There are other expensive stamps that should be certified as well, but I'm not home, so can't tell you which ones. Let the Scott CVs be a guide.

I've noticed that Scott #20 has a period after "8cts" which I don't see in the Scott catalog. Is this a variant, or a fake?

#20 has a thin 8 cts. surcharge with a period after cts.

This might be a good place to start:

http://www.canalzonestudygroup.com/index.php

This too is an excellent place to start: Canal Zone Stamps
PLASS Gilbert N. BREWSTER Geoffrey, SALZ Richard H.
Published by Canal Zone Study Group, 1986

There's a copy for sale here: https://www.googleadservices.com/pa...gBEA0&adurl=

Not only are there forgeries, there are countless varieties of O/Ps. Determining 14, 15, 18, 19, and 20 apart is simple when you have examples. Images in Scott are not the best.

As the editor of the Canal Zone Philatelist, I just this month published a nice piece in differentiating the 8c on 50c overprints. The issue prior was normal versus antique. If interested DM me or go to the CZSG site listed above and look me up on the site.