"El visto" is the first part of a resolution or report, at the beginning of the preamble, where the documents on which the resolution or report is based are cited. It comes before the "considerandos", so called because they begin "considerando", which are known as the "recitals". The "visto" is called the "citations" in English.

"visto
3. m. Der. Parte de la sentencia, resolución o informe que precede generalmente a los considerandos y en que se citan los preceptos y normas aplicables para la decisión." (DRAE)
http://lema.rae.es/drae/?val=visto

"9. Los vistos tienen por función indicar la base jurídica del acto, así como las principales fases del procedimiento que hayan conducido a su adopción."
http://eur-lex.europa.eu/es/techleg/9.htm
"9. The purpose of the citations is to set out the legal basis of the act and the main steps in the procedure leading to its adoption."
http://eur-lex.europa.eu/en/techleg/9.htm

Examples I've found of "carta del visto" from Peru refer to this. For example:

http://www.mindef.gob.pe/informacion/documentos/RM 357_2013.... (see "visto" and second "considerando")

http://www.regionpiura.gob.pe/documentos/resoluciones/phpSPn... (see "visto" and article 1 on p. 2)

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Note added at 4 hrs (2013-05-27 17:02:19 GMT)
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I'm assuming your document contains a reference to this letter at the beginning, probably headed "visto" or "vistos".

If the letter referred to here is the only document cited under "visto", then perhaps it would be better to say "the letter referred to in the citation above", or even just "the letter cited above".