Standalone "Cartas" is referring to business correspondence, whereas, in this particular case, "Cartas Socias" is referring to private correspondence or non-business letters.

Or would say 'Private or Personal (correspondence) Letters'.

Good luck :)

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Note added at 6 mins (2008-10-01 02:06:13 GMT)
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I (not 'or') would say, sorry :)

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Note added at 5 days (2008-10-06 13:12:29 GMT) Post-grading
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Thanks. One thing that is worth pointing out, as an afterthought, is that this type of personal correspondence is/was much cheaper in terms of mailing costs when compared to standard business letters. Hence, it's not only 'Private/Personal Letters', but 'reduced-cost private letters'.

It makes no sense (to me) charging less because of content type, rather than pricing letters based on destination and weight etc... A complicated and awkward system which has been largely outmoded these days because of the dissemination of the Internet and chat programs, and the like.

Best of luck.