Hi Dieter,
The catalogue doesn't explain why this was done but your suggestion sounds reasonable to me.
Here's the note from the catalogue (Higgins & Gage Volume A) in Argentina section D (Official):

As you can see, this note also includes two other envelopes from a much later period with a different styles of blocking (and the words "Sin Valor Postal" - without postal value).
This envelope appears to have been originally H&G B3a (wove paper) or maybe H&G 3 (laid paper) from the 1878/80 issue.
The suggested price of $7.50 is from the 1977 edition.