At a later date, one of the owners exchanged it for the beautiful green horn, which I haven't seen in any Gramophone Co. catalogue
I don't think this is true. Although the scan shows a Morning Glory horn of a pattern you and I are more than familiar with, the horn on Marcel's machine is absolutely original to it and correct. The Spanish HMV company got horns unique to their models (at least unique as far as I'm aware) and I believe they were made by Pathé or at least by the French company who made Pathé horns. They are the same gauge of metal and almost identical in shape and design. Remember also that it was the FRENCH division responsible for supplying the Spanish market and they might have sub-contracted out the production of "special horns". The colouring was a feature of these specific horns as well. HMV normally only gave red, black and green MG horns as an option. The Spanish market demanded far more decorative and elaborately painted horns. I have seen about half a dozen examples on the internet and they are all identical.
The Spanish machine I have has a cabinet that was probably made in Germany by the same company who made the Melba pedestals. Again, the horn on this machine is almost definitely French and is nearly identical to Marcel's. However, my machine being slightly later has "HMV" decals and the horn as an oval indentation in one of the petals with the dog and gramophone transfer within it.
Maybe, when these adverts were printed they were only expecting 'standard' horns on their models?