The Spanish sounds typically wordy and clumsy. Aren't they just saying that the original manuscript has not been completely preserved? Unless you need to keep "por la via de" to contrast with a different "via", then it may not be necessary to keep it.

-that's what it sounds like to me

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Note added at 1 hora (2009-05-21 08:58:21 GMT)
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CORRECTION:

"IN MANUSCRIPT FORM"

The St Fagans archive consists of an assortment of documentary material. As one would expect in a museum, the main emphasis from the outset has been placed on the collection of objects - in this objects concerned with folk life. However in addition to these, we have material in **manuscript form**, such as..
http://www.museumwales.ac.uk/en/193/

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Note added at 1 hora (2009-05-21 09:00:37 GMT)
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Therefore, having exposed herself to the public sphere by publishing her novel, Lady Hawke may have felt it was more appropriate to retain her other works in **manuscript form***. Although Julia de Gramont was Lady Hawke’s only published work, Lady Saye and Sele alluded to the presence of at least one other when she stated that her sister had ‘written two novels’13. Yet, there is no record of this second work ever having been published. This would indicate that the novel must have remained as a manuscript.
http://www.chawton.org/library/files/Hawke1.pdf

throughout (a lo largo de) -> means that some of the text may be deteriorated in parts...

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Note added at 3 hrs (2009-05-21 11:00:12 GMT)
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"... original is not completely/fully/ etc preserved throughout the manuscripts ..."