Thinkstamp, I wasn't aware of that article - too bad it's not freely available online. I'll have to track down a copy for my Heligoland reference file! Thanks for sharing, and looks like you've put together a nice collection.
All articles in The American Philatelist
are available on the Robert A. Mason Digital Library: https://digital.stamplibrary.org/digital/.
Access to the library is free - instructions to log in are available on the site.
I encourage you to search for the article and add it to your collection.
We are also uploading Lawrence Mead's exhibit to the APRL Reference Catalog, which is also available to all.
We continue to add new information to both the library and reference catalog.