Hi Sebastian!
That was LU5BOJ/O from FG75 whom I worked at 01:52 UTC. We worked briefly as he was about to lose the bird. Not sure why he used the portable designation. It was O for Oscar as per his phonetics, and not a zero. Perhaps it's required in his country while working through a satellite?
You're 100% correct.
That callsign LU5BOJ/O would be a valid call in Argentina. Unlike in many places like the continental US where the digit indicates the area where the call was issued, in Argentina the first letter after the digit is used for that. LU5BOJ is a call issued for the city of Buenos Aires (also known as "Capital Federal"), separate from the province of the same name that surrounds that city. It is like how the US capital city Washington DC is separate from the surrounding states of Maryland and Virginia. The "/O" shows he was operating from somewhere outside his home "province"; in this case, LU5BOJ was in Salta province in the northwest of the country.
You can see a map and how those indicators match up (thanks to my friend Diego LU8ADX) at:
http://www.lu8adx.com.ar/mapa/index.htm
That's the first station I have heard from Argentina on any of the birds.
Congratulations! Maybe more will show up on HO-68 or even AO-7 if equipped to work the linear transponders.
73!
Patrick WD9EWK/VA7EWK http://www.wd9ewk.net/
(I was down there in 2004, operating briefly as LU/WD9EWK. Foreign hams do not have to put the indicators like "/O" after their calls to show where in the country they are operating from as the locals )