Canal Zone 18B Vs. 20

I have a couple of early Canal Zones, and one that is labeled as 18b. However, I am trying to figure out what makes an 18b different from a 20 (or for that matter a Panama 74b different from an 81 which is the stamps used). They appear to have the same overprints, however on the canal zone 20 (panama 84) one is up and one is down as compared to both being up. That is the same case for the 18b however. Is it a matter of which side is up and/or down (e.g., left down, right up vs. left up, right down)?

THanks

Michael, according to Scott there is a difference. On 18b Panama reads down and up. On 20 it reads up and down, the other way around. Could you post a scan of both stamps?

Peter

Thank you - so that's what I thought, it is there direction of left vs. right. I will try to post a scan later, but basically both are up on the left side of the stamp, and both are down on the left side of the stamp, so it looks like the 18b is misidentified. THx.