A very similar counterstamp - but not on this coin - is listed by Gomes in Moedas Portuguesas, ISBN 972-97804-1-2 (2003 edition) under Moçambique, D. Carlos 01 - 09. Gomes notes (Google translation from the Portuguese original):
This stamp began at the end of the reign of D. Luis I, replacing the crowned stamp, and for the same purpose.
Currencies of another type or dates that are not indicated in this relation, will have the same value, relative to the prices indicated above. When the stamp occurs in coins dated after 1895, it is at least suspect
Which shows a) there are similar counterstamps on other coins and b) there are forgeries. Take into account that the counterstamps were applied 1888-1895 and the host coins listed are dated 1835-1892. Your coin seems an early outlier. On the bright side, most coins shown are British Indian, including several rupees, that were, however, lighter than your coin.
Peter