The Argentine name for the Pneumatic Tube Mail is "Expreso Urbano".

#1 - Teilsending von rohrpostrohren zur verladung nach Argentinien 
"Part of a shipment of pneumatic tube pipes for transport to Argentina"

#2 - C. Aug. Schmidt & Söhne Pneumatic Tube stations at the Telegraph Office in Buenos Aires

Photos #1 and #2 found on page 102 and 99 of the book "100 Jahre C. Aug. Schmidt Söhne, Hamburg".
The book was published in 1941, therefore the photos are believed to be in the Public Domain.

#4 - Envelope Buenos Aires 19 april 1934
Stamp at the backside: "Inauguracion del servicio neumático ... 1934"

Sources and more information:

• Wikipedia - Rohrpostnetze in Amerika, Buenos Aires [D]