I think "S/E" probably stands for "sin especificar", and what it means is that the identity type is unspecified. In other words, whether the ID number is that of a libreta cívica they put LC before it, and LE for libreta de enrolamiento, and so on; but when the type of ID document is not specified, they put "S/E".

The following example is from a different context (health services rather than birth certificates), but it may reflect relevant standard practice:

http://www.msal.gov.ar/pngcam/resoluciones/msres172_1997.pdf (see p. 37).

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Note added at 1 hr (2012-10-22 16:54:22 GMT)
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Sorry; it wouldn't let me copy and paste the text from the document just cited, so I've got to copy it out:

"Nº de documento
Anotar el número que figura en el documento presentado por el paciente.

Tipo
Indicar el tipo de documento presentado por el paciente
DNI = Documento Nacional de Identidad
LE = Libreta de Enrolamiento
LC = Libreta Cívica
CI = Cédula de Identidad
Otros = Cédula de identidad de otro país, certificación en trámite, pasaporte, etc.
S/E = Sin especificar tipo de documento
Anotar

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Note added at 1 hr (2012-10-22 16:56:28 GMT)
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In the first line of the explanation above, I meant to say "the identify document type is unspecified".