Hello GUEST< JTT, you asked for "the words of the Spanish Civil War song La Guardia Rossa ". This song did appear in a publication used by the International Brigades in Spain during the Spanish Civil War called " Canciones De Las Brigadas Internacionales " and was therefore probably sung by International volunteers at that time. The song is printed on page 71 of a reprint edition brought out in Spain in 2007. The words to the song are very close to those given above by
Jim Dixon but with a few words differing on some lines. For example,
in the book the first verse begins

Ecco s'avanza uno strano soldato,

instead of
Quel che si avanza è uno strano soldato,

and the verse ends with the the lines
Porta la falce e il Martello incrociati:
Gli emblem del lavore.
Viva il lavor!

instead of
mostra un martello e una falce incrociati,
son gli emblemi del lavor. Viva il lavor!

Verse 2 of Jim Dixon's version appears as verse 3 in the book and Jim's verse 3 is printed as verse 2 in the book. Again , occasional words in these two verses are slightly different
The final verse 4 in the book is I think essentially the same as Jim Dixon's version but again there are some slightly variant words.