Dear Members, I noticed that this forum has been dead for multiple years, but there are so many knowledgeable members here that I wanted to see if anyone can point me to an answer.
I am very new to this fascinating hobby and the more I learn, the more I enjoy it.
I recently joined the Guatemalan Philatelic Association and they have a very active WhatsApp group and I asked the question of which was the first "official" (sold by Correos de Guatemala) FDC? Silence followed, but then multiple people starting sharing their 1940's FDC's but we never got to the conclusion of which one is the first official. We came across a few from 1937's (US 150th constitution anniversary ) 
But we cannot find if it is an official envelope or just a cachet created by some private philatelic vendor.
Same happened with the 1938 Central American Philatelic Exposition:

All meet the requirements to be a FDC but not necessarily be "Official"
Also came across the Memorandum of 1930 thanks to a member here but we came to the conclusion that doesn't qualify to be called FDC , just a Commemorative Envelope and also first Official Cachet from Correos de Guatemala:

I also looked into the First Airmail covers but all look like commemorative cancellations over old stamps.
The last one I found (and I don't own), is the following a 1929 "Inauguration Ferrocarril Oriental" that again, has some sort of special design and it does qualify as a FDC, now I need to see if its an official envelope:

Please share your thoughts, ideas, or where should I keep looking, all help is appreciated and if you have an Scott #182 also let me know, I need it to complete my Collection :)
Carlos