Without a guarantee for authenticity from the
ebay source, this item should be taken personally to a numismatic professional (such as Heritage), for an appraisal.
With high value at stake here, a numismatic professional would NEED to look at the coin in hand.
By their very nature, most patterns are extremely rare, and I for one, do not have the expertise to be able to give this item the

or

, despite the fact that I already have a few authentic patterns in my collection,
(appraised and agreed on by two of Australia's leading numismatic professionals - who are public auction lot describers for leading Australian numismatic public auctioneers).
Needless to say, I am unfamililiar with this particular pattern, therefore I cannot comment.
A Google Images search for
'Argentina pattern peso 1880', yielded an appropriate image:
KM#Pn20.