I will try to find that posting on the website, as I can’t get much out of the normal poor “Babelfish” translations. My Spanish is not fluent, but it is enough that with my dictionaries I can translate something short without much problem, and with considerably more accuracy than babelfish.
I can’t comment on whether Electra made the 7 x 57mm or not. It is out of my field and I have never seen the round to know its characteristics. However, I find the entry in the Small Arms Identification guide suspect. Firstly, it is an American Government publication on ammunition. Aside from specific reports, with the actual manufacturer’s input or the input of those testing a specific type of ammunition, which are usually detailed and, I assume, accurate, I have found U.S. Government publications on general ammunition themes to be sadly wanting for accuracy and completeness. The 9 x 19 cartridge was not widely used in Spain until after WWII. If the Guide referred to this as a 9mm Largo (9 x 23mm) I would be less suspicious that it is wrong, but I don’t know anyone who has seen either caliber of pistols ammunition mentioned with this “M” headstamp. That doesn’t mean it was never produced, but it does call it into question.