I'm just as confused as before.
sometimes, we have to step outside SCF, and search for specialist knowledge, this, I believe, is being a "Philatelist"
Then come back, and impart your learned knowledge here.
Information on Cuban revenues is very difficult I find, I take some information, from the Philippine example, and by Fulcher.
1938? perhaps a difficult time, and mixed franking was tolerated for revenue?
Nice covers, worth taking the journey to research.