Aha ! The correct flag was a flag without a monogram, which corresponds to good flag design principles. Obviously, I was wrong about that monogram. Only much later, I discovered other Trasatlantica certificates from the 1940s depicting a blue flag with a white disc.
This share left me another surprise in regards of its flag. It has a remarkably large watermark of the house flag.
When holding against a light source,
a watermark of the flag appears measuring about 12cm x12cm
The Latin word vexillum means flag. The study of flag design and usage is called vexillology.
More interesting reading material about this topic and the history of the Compañia Trasatlantica can be found on the following web resources :
- House flags of shipping companies
- Funnels, Flags, and Night Signals of the Transatlantic Lines
- Flags of the World
- Compañia Transatlantica Española
- Flags of shipping companies with wrecks
Embossed seal depicting sailing ship,
yet another feature near the lower border at the left.
(click image to enlarge)